Ops, I have missed that is actually WORKS with MT... (thought it is otherwise).
Well, this looks like very likely new hypothesis:
KDE is multithreaded and needs multithreaded memory allocator
Actually, this could even explain some mysterious MacOS behaviour we have encountered recently too...
Anyway, this should USEMALLOC fix too (then it would use regular malloc and everything should be OK). This is really strange, with USEMALLOC, that mcache variable is #ifdefed out of code...