Use a debugger.
This is an old bug within the facelets library bundled with seam 1.2. We have exchanged the jsf-facelets.jar with version 1.1.13a which fixes the bug. I think you can use facelets 1.1.14 (currently shipped with Seam 2.0.0).
This fixed the problem, and 1.1.14 does not seem to cause any issues with Seam 1.2. Thank you so much for sharing your experience with me! This is also what makes a good framework: A good community.
You can't imagine how glad I am now that this nasty bug is fixed and I'm not sure if and when I could have fixed that without your help.
Thanks again and I really hope that I can also help you some day.
All the Best