In my case it always happens in pymalloc_alloc when size == 5, e.g. when accessing usedpools[10].
Specifically freeblock pointer in usedpools[10] is waaaay off (essentially, seemingly random number looking like 0xXXXXX000) where it is supposed to be (e.g. very far from usedpools[10]). Surprisingly, all the other fields in usedpool[10] seem to have normal values.

I could not find any place in obmalloc.c where this value would be set, so it seems to be a memory corruption caused by something else.