This has been happening to me continuously the last few days. The auto save will start failing after an hour or so and when I try to save all or close the project I get the same error as you.
Does it let you just perform the save as Untitled and continue? For me it will infinitely prompt me to save a new level as Untitled. I tried seeing how many times it would keep prompting me and I ended up saving 12 levels before I just force closed the editor. I can manually save the actual level fine but it’s really annoying getting a giant message log on my screen every time it tries to auto save and getting trapped when I try to close the editor.