The same symptoms seem to occur under Websphere Application Server 6.1 on z/OS. Runs fine under WAS 5.1 on z/OS.
The application also uses many forwards (using Struts tags), but if I would include the "avoid filtering code", what would be the use of the filter? I assume the most part of the application then wouldn't be filtered/compressed at all.
Did you find another solution to this?