TWO HOURS. The fucking mission was TWO HOURS long and DIDN'T LET YOU SAVE in the middle and then FUCKING CRASHED and oh guess what, the most recent save I can load is before the PREVIOUS FUCKING MISSION which automatically chained into it!
...which crashed the first time I did it so I've already done that previous fucking slog of a tedious shit-hole of a mission twice. NEWS FLASH -- ENDLESSLY RESPAWNING ENEMIES == NOT FUN. NOT FUN * 3 == FUCK.
Okay, now that I calmed down (it took a while) I realize what happened -- the first crash left some sort of lock on the save file directory, so nothing could be saved after that. I should have realized this when I saw the GOG launcher failing to cloud-sync the save files. Probably could have fixed it by rebooting.
The game DOES let you save during that mission and probably autosaves at the start like with every other mission.
That still really sucked though. x.x