Java Web Application Error - Unable to understand why it's happening now

I am using eclipse and I can build the webapp and deploy it to Jboss4.2.1. It was working previously and I haven't changed anything really extensively to make this error come about. When I go to the webapp thru my browser, I previously have been signing in with a generic username and then I could use my webapp as usual.

Now when I go to the webapp thru the browser, this error occurs repeatedly and will not show anything. Here is the error that I am receiving:

07:55:06,174 INFO  [security] User '' attempting login via SiteMinder...
07:55:06,174 INFO  [security] User '' attempting login via SiteMinder...
07:55:06,174 ERROR [[jsp]] Servlet.service() for servlet jsp threw exception
java.lang.LinkageError: Class javax/el/ExpressionFactory violates loader constraints
    at org.apache.jsp.WEB_002dINF.jsp.layout.header_jsp._jspx_meth_c_005fset_005f0(header_jsp.java:207)
    at org.apache.jsp.WEB_002dINF.jsp.layout.header_jsp._jspx_meth_c_005fif_005f0(header_jsp.java:179)
    at org.apache.jsp.WEB_002dINF.jsp.layout.header_jsp._jspService(header_jsp.java:82)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:387)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:687)
    at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:590)
    at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:505)
    at org.apache.jasper.runtime.JspRuntimeLibrary.include(JspRuntimeLibrary.java:968)
    at org.apache.jasper.runtime.PageContextImpl.doInclude(PageContextImpl.java:643)
    at org.apache.jasper.runtime.PageContextImpl.include(PageContextImpl.java:637)
    at net.sourceforge.stripes.tag.layout.LayoutRenderTag.doEndTag(LayoutRenderTag.java:122)
    at org.apache.jsp.WEB_002dINF.jsp.layout.layout_jsp._jspx_meth_stripes_005flayout_002drender_005f0(layout_jsp.java:796)
    at org.apache.jsp.WEB_002dINF.jsp.layout.layout_jsp._jspx_meth_stripes_005flayout_002dcomponent_005f2(layout_jsp.java:759)
    at org.apache.jsp.WEB_002dINF.jsp.layout.layout_jsp._jspService(layout_jsp.java:200)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:387)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:687)
    at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:590)
    at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:505)
    at org.apache.jasper.runtime.JspRuntimeLibrary.include(JspRuntimeLibrary.java:968)
    at org.apache.jasper.runtime.PageContextImpl.doInclude(PageContextImpl.java:643)
    at org.apache.jasper.runtime.PageContextImpl.include(PageContextImpl.java:637)
    at net.sourceforge.stripes.tag.layout.LayoutRenderTag.doEndTag(LayoutRenderTag.java:122)
    at org.apache.jsp.view.public_.login_jsp._jspService(login_jsp.java:335)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:387)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at net.sourceforge.stripes.controller.StripesFilter.doFilter(StripesFilter.java:180)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.springframework.orm.jpa.support.OpenEntityManagerInViewFilter.doFilterInternal(OpenEntityManagerInViewFilter.java:112)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:265)
    at org.acegisecurity.ui.switchuser.SwitchUserProcessingFilter.doFilter(SwitchUserProcessingFilter.java:341)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
    at org.acegisecurity.intercept.web.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:107)
    at org.acegisecurity.intercept.web.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:72)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
    at org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:110)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
    at org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:110)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
    at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
    at org.acegisecurity.wrapper.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:81)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
    at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:229)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
    at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:229)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
    at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:286)
    at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)
    at org.acegisecurity.util.FilterChainProxy.doFilter(FilterChainProxy.java:149)
    at org.acegisecurity.util.FilterToBeanProxy.doFilter(FilterToBeanProxy.java:98)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
    at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:179)
    at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:84)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
    at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:157)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:241)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:580)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
    at java.lang.Thread.run(Thread.java:595)

Any help would be greatly appreciated.

The runtime classpath is likely polluted with duplicate and/or different versions of EL API.

Normally, the EL API is already shipped with servletcontainer/appserver in question. You should in no way copy/duplicate/move its libraries somewhere else, such as into webapp's /WEB-INF/lib or more worse the JRE's /lib. Cleanup the classpath from all duplicate and/or different versioned classes / JAR files. It may otherwise lead to exceptions like this.

A common mistake among starters is namely that they copy the servletcontainer/appserver's libraries into /WEB-INF/lib because they got compilation errors in the IDE (javax.servlet not found and so on). Copying the libraries is the wrong solution. You should rather have integrated the server in the same IDE and configured the IDE project to reference/associate the server in question so that the IDE will automagically take its libraries in the project's buildpath.