Confusing Segmentation Fault when using Threads

Following Problem:

I initialize my Program (InitSDL and stuff) create a new thread
(purpose: Network handling) everything works fine (i presume) so far,
then i start creating some objects (in the main thread) with “new” and
then suddenly one of the "new"s crashes with a segmentation fault…

when i don’t start the thread, everything works fine exept the network…

All this occurs while running on Linux (Mandrake Distribution)…
when i compile the prog for windows XP it seems to work…
Win 98 crashes too…

Anybody ANY idea what kind of coding mistake i am supposed to look for?
I’m kinda desperate…