In case anyone has this problem:
I had this issue because I had deleted a bunch of streaming actors when they were unloaded, and then I recreated them. old files existed still in folder structure, so I couldnt save newly created ones, which for some reason used same names.
old files which cause fail to save may exist already in folder:
“…\Unreal Projects\ProjectName\Content_ExternalActors_\Levels\LevelName\X\XX”
I checked date and time of one of left over old files.
Then I used windows search and deleted everything created from and after that time.
Then I was able to save my project as normal.
Might cause issues, do at own risk 