Indeed, Time Machine backups over network to a host other than a Mac don't work any longer. I hoped the hint I gave myself in the last post would help, but actually it didn't. This is what an apple engineer said as an answer to a bug report (source):
"Network backups to anything other than another Mac OS X Leopard box or a Time Capsule are not currently supported. Time Machine preferences should not have even offered to use a SMB volume as a backup destination. We can't speculate on why it may have appeared to work on 10.5.1, but not on 10.5.2."
Wasn't Time Capsule presented just some weeks ago? No, there CAN'T be ANY link between this presentation and such a "bugfix" ...
And something positive: I finally have DSL here. "Only" about 6000kbit/s but it's much more than with UMTS or DSL-Light which I only had at home in the deep Eifel forests.