When a Seam managed resource produces an exception the exception is swallowed, i.e. it is not logged. Furthermore, maybe this is also the source of the problem, the failed request is not redirected to the correct debug.seam URL.
Steps to reproduce on JBoss 5.1.0.GA with Seam 2.2.0.GA:
1. create a Seam resource that produces an exception (code attached)
2. navigate to the Seam resource: /seam/resource/myresource
Now instead of redirecting to /debug.seam?cid=<CID> the browser redirects to /seam/resource/debug.xhtml?cid=<CID> which of course does not exist, thus resulting in a 403 HTTP error.
Furthermore the exception is not logged in the JBoss logs.
When you manually navigate to /debug.seam?cid=<CID> the full exception trace is shown (serves also as a workaround).
Example resource (requires existence of resource servlet in web.xml) is attached.