Thank you for your replies. Because it wouldn't work with different browsers, I didn't expect the problem to be on my side, but it obviously was ... and I don't understand it *

*
neno.tu, following your suggestion, I logged into my account at university and started firefox there, which was awfully slow

but well, it worked. So I tried and did a system update. Still no success. Then I dropped my ppp-connection and reconnected and voilą. The puzzling thing about that: I have a reconnection every night and it didn't work for some days...
Well, I won't try to understand. Now it works and I'm fine, sorry for the false alarm!
Greets, Felix
edit: this is _really_ weird: it worked one time, but now I just get the same errors again.

wtf?