D2GS Random Crashes

Two addresses in your Stack Crawl can appear in many situations (try google it):
6ff60eaf
6ff612eb

If your server is attacked through a vulnerability then it also should produce the following line in D2GSLib-Debug.txt:

D2GSErrorHandle: Unhandled exception occured

But not it your case. So, it should be your patch problems.

I think google of this address will not give you additional info about the issue, because it appears in very many situations, including not only a server but also D2 client.
Actually, you should look into a stack dump in your D2-161209.txt files to find a crashed module.
As I understand, Fog.dll is the end of trace, and the start is D2GS.exe in your trace.

You should know better which and where modifications were made, and which of them may be unstable.

Earlier I though that incorrect patch_d2.mpq may be a reason of a client crash only. But a client and a server uses the same dlls which use data from mpq, so may be it's possible for a server too.

Do you use an original D2GS by Marsgod with only patch_d2.mpq modified, and which version? How often the crashes occur?