Hello, please make sure that the game has necessary permissions over your security software, otherwise the game can not create save files on your PC. Please let me know if the problem persists after completing this step.
I've experienced this issue several times
without any 3rd party Security SW installed.
This happened with modded and unmodded games.
I have Bannerlord under the "exclusions" list for Windows Defender since day one

see below
Appreciate if the devs have any insights if anything else could be done on the Windows side to alleviate this.
Also of note - my Bannerlord installation is
NOT under the C: drive - would that be an issue/clue worth pursuing??
Been unable to replicate this issue since 1.3 became the latest stable (I've started a fresh playthrough).
But as you can see in my post above, there's something that makes savefiles size become "bloated" as the game goes on.
The issue consistently comes up once files pass the ~80Mb threshold, but it also happened before that.
There was one occasion however where brand new campaigns with no mods had this issue and I wasn't able to save at all.
Later I found out that it was probably Steam's fault breaking game files between updates (a "verify game files" had me redownload a few gigs of data).
Would it help you this older branch savefile that present this issue? (1.2 beta IIRC?)
drive.google.com
(albeit this one was full to the brim with mods, I can't even tell you which mods I had on)