There isn't a lot of information to work with here. However, one thing that's important to keep in mind is that EL was not built into the Tomcat server until Tomcat 6. Webapps for Tomcat 5.5 and earlier had to supply their own EL jars in their WEB-INF/lib directories.
It is therefore possible that the app was built for an older server and you are experiencing class conflicts with the EL classes in your server's built-in libraries.
An IDE is no substitute for an Intelligent Developer.