The problem seems to be solved now.
I think i was a bit blind again... In the rare cases the system "un-freezed" after removing the card, the tulip_cb module was still used (and thus not unloaded), and all subsequent card insertions did not use the ioport 0x200 used upon first card insetion but port 0x280 instead.
So i just changed the config to not use the port range from 0x200 but from 0x280 and now all works. I am still not sure what conflict i ran into, but whatever it was, it is in the region 0x200-0x27f.
Best regards, Wolfgang