Dear Andreas, thanks a lot for your report.
The commented fail usually is solved executing the "uDig -clean command".
I see you are using a very old release, it is "rc" (Release Candidate), this kind of release is done for testing purpose. So, I suggest you to update for getting a stable release. Right now, in the "udig update site" is available 1.1.4 (alternatively it is available in the downloads page http://www.axios.es/
Let me now if you have success.