Happend to me as well - solution which worked for me is below.
Targets:
- Avoid reinstall of the game
- Keep all players settings in save untouched
- Situation description
- Game was modded (> 80 mods), but with fully functional saving system
- Suddenly without any change in the game or Nexus, the game was crashing to desktop without any information during saving
- save files were suddenly saved as *.fos.tmp instead of *.f4se and .fos (save files in C:\Users*user\Documents\My Games\Fallout4\Saves)
- solution, which worked for me (chronologically)
- Created back-up of saves
- Deleted cca 50% of obsolete saves in Fallout 4 save structure
- Switched off all "Plugins" in Nexus (Plugins, not the mods!)
- Started Fallout 4 game
- Load last game and save as a new save for FIX ONLY. Suddendly saving worked without crashing to desktop (FYI this FIX save lost all tracks to plugins)
- Closed the game
- Switched on all "Plugings" in Nexus
- Started Fallout 4 game and opened my last FIX save (all plugins are started now, but the FIX save is not as it was in original save - e.g. tatoos are removed) and save the game, which worked without crashing to desktop again
- Loaded my original and target save (which I want to really use for gameplay) and suddendly the saving system is functional again (and also complete player settings is kept, because the original save remembers it)
Key take away: note #1: FIX save is created as help to make saving functional, for gameplay non-deformed save to be use (= player settings, look, etc. are kept). note #2: during process were switched off PLUGINS in Nexus (not mods!) note #3: I susspect the amount of save file in original folder as well, when it started I have >600 saves - I´ll keep eye on it. The problem re-occure again after 2 weeks, but seems to me that problem can be in save folder (?). note #4: the whole process took for me max. 5 minutes
I hope, that it was helpful for someone.