well, I looked through the code again, and it seerms very unlikely that there is a problem with the mutex object itself (among other things because in order to reach the assert, the queue type must be queueQUEUE_IS_MUTEX which is unlikely for an overtrampled pointer).
Thus the only possible scenario for the assert to catch is when pxCurrentTCB gets overwritten. I would probably dump it periodically to see if that looks like a valid pointer.