I am converting a 100+ BIAB .MGU files to .XML. Some files fail to export and will yield an error message that reads:

"Error creating this filename ? ://Invitation.XML"

I have tried many work-arounds, but am very frustrated by this. Sometimes it works properly, but I have not been able to determine why. Has anyone else had this problem.

I am on Build 186. It actually seemed to work better (?) a few builds back.

Then why not reapply that older patch while you're completing this specific task and then reupdate to the latest build? Just a thought, since going back and forth with patches is usually quite safe and simple.
Also, it may be that something starts to hiccup due to the share number of files you're submitting to this conversion. Maybe doing something like 50-70 per session will work more steadily.