Gosh, I was just too stupid.
E-Mail did no longer worked and I found an error
No RenderingContext has been created.in the log, so I assumed this to be the cause.
However it's really only a message than can be ignored.
The real cause was that while migrating from Trinidad 1.0.6/Seam 1.2.1 to Trinidad 1.2.7/Seam 2.0.2 I somehow lost the jboss-seam-mail.jar.
There could better have been a message from Seam about missing classes/components, though.