Found the reason why I'm getting a segfault. I didn't keep in mind how a ressource's ressource manager pointer was managed when multiple threads access - quite delicate since I deemed it done myself! There's no clue whether this is also the reason for qmutex relock problem but well - eliminating this one can only make the other error more clear. It's slightly reproducable. There are a few situations which definitely produce this error, so I'll can locate it more easily. Seems that I'll have rework this one a bit but I'm very glad that I found this error. It's definitely a null pointer or, maybe in the case of freezes, something else pointing to where it doesn't belong.
Woohoo, we got a clue! That's good by default, I think.