I'm not sure it's known, but theres this bug that happens if you modify a classic character on GoMule while it's on Nightmare dificulty, take this save i attached for instance, whem hes on nm and you delete something from his items or even pickup and drop again at the same grid slot it breaks the save file: can't load again either on D2R 2.7 or GoMule R0.44 (4.4.20-BETA) - changes start to show after . I already checked for the expansion exclusive flags on the save if that was the problem, now that i mentioned, if that was the case it would break on normal just aswell - which does not happen! If you load that same file on a normal game with the client and save/quit then you can manipulate his inventory on GoMule, save everything and it's all fine.
Classic Character: one created on D2R to play pre-expansion content (up to act IV) - just to be clear.
I have the same symptoms (also on classic) but I haven't even reached Nightmare yet.