Ignacio,
I have not found (nor, really, looked really hard) for a solution to that problem. For now, the customer has discarded JBoss 6 as an alternative since besides this issue, it's not supported other than by the community, and this is insufficient for their needs.
Importantly, I recall finding a JIRA ticket describing the issue, and that it would be fixed for 6.0.1 (release on/about March 18 next).
This seems to be related to the use of form-based authentication. If you're using that, you might try switching temporarily to BASIC authentication to see if that fixes, or at least allows you to further isolate the problem.
Cheers.