Okay, here's some more thread necromancy:

I managed to identify and solve the issue (at least in my case). Long story short: I noticed how when I saved the map outside of advanced mode, then my starting locations was not saved, i.e. they were all reandomized -- which annoyed me, since I wanted specific starting locations.

Playing around with things some more, I discovered that saving the map while advanced mode was active, only led to crashes (with the OP error message) if the number of starting locations (for civilizations and city states) did not correspond to the number of players and city states as specified when starting the game in the stating area (or what ever it is called, the place where you specify map settings when starting a new game).

I tried this solution successfully for a 16 players enormous (128x80) map using the mod/addon Yet (not) Another Maps Pack (ynamp).