old thread, I know, but google yadayada …
In my case it was caused the QNAP QSync service (backup agent).
I had the same SaveGameToSlot issue, and just wasted three days investigating, debugging, test printing, …
Sometimes it helped to just try SaveGameToSlot like 200 times in a loop, until it worked, but I found no pattern.
The solution:
By simpy killing the QNAP QSync agent all SaveGameToSlot issues immediately vanished, and it worked as intended.
Took three days to recall that I had similar issues e.g. when trying to rename files or folders in the file explorer while that service is processing file in that folder; first it refused the action, but after some time it worked.
The local QSync client constantly scans a configured local path and instantly copies changed files to the QNAP NAS and seems to lock them during that time.
In my case I tagged the whole documents folder including the unreal projects for syncing, causing it to also process the UE sav files.
As I do standard backups anyway, I’ll ditch QSync now as it seems to cause more issues than it prevents. Not recommended for professional environments.