JBoss Application Server 5.1.0
Richfaces 3.3.3 (I also tried with richfaces 3.3.1).
JBoss Seam 2.2.0
JBoss Tools 3.1.1
OS: Debian Squeeze AMD 64
I can solve this problem by creating another project and importing the files of the last one, but sooner or later this error happens again. I already asked for help in Seam forum, but nobody tried to help me. I'm almost giving up of using jboss tools for now.
Sometimes the pages get back to normal, but the same way the error comes back.
Could you check if all files were deployed correctly to the server after you made changes in your project?
Also I would suggest "Full Publish" through server View + Clean Cache in Browser + Refresh, just in case you haven't tried it yet.
.xcss files have xml format so for validation you can use regular xml editor.