Okay, I was experiencing a crash when loading The Ratway after completing that first thieves guild quest. Google gave me the hunch that it should have a clear-cut solution. You nailed it, my friend. I do not run OBIS (I hear good things though) but waiting 10 in game days resolved the issue. Some kind of f-ed up spawn going down in there and waiting for the cell to reset clears it up. (I used Extended UI and waited 240 hours at once.)