hmm, I can confirm this. I did update my JDownloader as well, as the updater became really annoying. I start JD from Console(Debian Squeeze/KDE4). It hangs when loading syntheticaWhiteVision.jar . I removed that file to trigger an error(sry, logging disabled :-().
after that it hangs with substance.jar, which is just before synthWhiteV, so I guess the error is after that.

The updater fails with no error, it just hangs at "starting..."
Trying to restore fails just the same way.
I also noted that the update-servers that the update.jar prints on stdout all send 404 (Not Found) when entered in my browser.
Overwriting the JDownloader Folder with a fresh copy from zip does not fix that problem.

I also did download a fresh copy and installed that separately, it won´t start either