Just wanted to let others know I received the "diabdat" mpq file error then a CTD when launching after that. This worked for me and game has been running flawlessly for me since.

Open task manager and end ALL tasks specific to Diablo and GOG Galaxy. Diablo 32 bit was one and there were 3 or 4 for GOG. End all of these and close task manager. Relaunch Diablo. May not work for all but it did for me. Hope it helps. Most probably tried this but it may help somebody. Seemed to me game launch was "stuck" and this unstuck it so to speak.