For others maybe having this problem:

Running BG3 with patch 3 (and mods), resuming from a savegame made pre-patch-3, one can perform one save, but after some thing / event happens (possibly interacting with a merchant), the game can not save anymore. Regular save shows a "Save Failed", while quicksave shows "Quicksave failed".

I got my game to allow saving once more by talking to a trader (Larroakan's non-person projection in Sorcerous Sundries), starting an exchange via the conversation option, switching back to the Barter mode (I usually stay in Trade mode), performing a Barter, then exiting the conversation normally. [for good measure, I then talked to the projection again, used the lower-left button to start the Barter, which was in Barter mode already, then exited]

Now I can save once again.

Speculation: since now the game tries to remember if one was in Trade vs. Barter mode, maybe the game can't save if it was in Trade mode already, and it saved that already, and it can't re-affirm that in subsequent saves? ¯\_(ツ)_/¯

Yeah, I know Larian does not offer support for the combinatorial space that is modded BG3 (don't think any studio could...). That's why I'm not asking Larian for support; I'm just sharing my workaround with the community.

Besides, people elsewhere (Reddit, Steam) seem to be having the issue in un-modded games, so maybe it's useful for some vanilla-purists too :p