There is a conflict between JBoss3.x-Tomcat4.1.12 and Struts 1.1b2 (specifically Tiles) which a number of posts have attested to. The issue is related to classloading and the use of the org.apache.commons.digester.Digester class. There are two ways to fix this:

1. Download the commons-validation source, and add the following line to org.apache.commons.validator.ValidatorResourcesInitializer after line 143:
digester.setUseContextClassLoader(true);
Then rebuild commons-validator.jar and repackage your app.

2. Download the attached commons-validator.jar with the above fix and repackage your app.

Please email me directly if this doesn't worl (lmagee@biziworks.com.au).