It’s been a long time since I ran into an AE error like that so I don’t recall specifically what my issue was, but three possibilities come to mind and the solutions are pretty self-evident:
(1) The file path or file name could be be too long or use some strange character that is confusing AE.
(2) There is some kind of write issue that could come from any one of several scenarios: you’re on shared storage and someone else has that file open making you unable to save it until they close it; you have a write issue with that particular file (permissions issues on Mac or file marked read-only on Windows); you have some other buggy software running that thinks it’s using the AE file
(3) The project has somehow become corrupted. Unfortunately, I have yet to find a good solution for this one, but I’m okay with that since it has rarely ever happened to me and even if it did I always save multiple copies of my projects on different drives so I’m never at the mercy of a single file of my project.