Debug Assertion Failed Error

I agree with S_M_A, in the sens of type of bug._ASSERTE(_BLOCK_TYPE_IS_VALID(pHead->nBlockUse));This assertion fails most likely because of a memory corruption. A buffer overrun is a common scenario, but can be also a thread syncronization problem. When the assertion fails, load the application into debugger and check first how the callstack looks like and which thread fails to free memory. It should help finding the statement which tries to discard the corrupted / already discarded buffer. Considering the multithreaded environment in which is running, review the syncronization for those kind of shared allocations. If the faulty code is within the method of a class, inspect the instance for which the crash occurs... might be an instance previously freed by another thread. You know best how your code is supposed to behave, so try to see what doesn't look wright.
If you found the type of object for which the assertion fails, a simple method exists to find the reason. Insert a signature DWORD as first member data of that class, which gets initialized in every constructor with, lets say 0x0B0A9776 ^ (DWORD)this. In the destructor, replace the value of the signature with, les say 0x13377B0A ^ (DWORD)this, marking the memory of the object as "deleted". The first thing that you should do in the destructor is to check if the signature matches the value assigned in the constructor. If not, means the instance should not be deleted, and to know if was already deleted, test the signature value for the value that your destructor is setting. If the an object is deleted twice from different threads because of a syncronization problem, that usually occurs almost immediatelly, thus you should see a signature of a deleted object, because it is unlikely that another memory allocation took place on an overlapping memory. If the signature value is sort of bogus (none of the 2 your code is doing), then most likely your code is corrupting the memory through some sort of buffer overrun.
Hope it will help in finding your bug.
All the best,

You can be sure that debugger is not causing any asserts in your code.

As said before, the most likely origin is that your log function writes outside a buffer. Add your own assert to verify that the log function behaves as expected and run in debugger again. When/if your assert kicks in use debugger and call stack to find out why.