BUG: saving unknown error
I'm on the newest version of Premiere Pro... windows 10 with a Puget PC specifically designed for Premiere. Most of my projects are very small and short, but when I have big projects that take several weeks and tons of media and editing, the bigger the project gets, the more saving fails. See attachment... I've been getting that error multiple times the last 3 days. That's why the file name has the letter K in it... I just keep Saving As with a new letter every time I get that error.
Similar issues also happened around December of 2019, when I last did a large project. The bigger the project got, the more it failed to save. I also ended up with an alphabet soup of project files for that one.
I don't know for sure, but I think the problems may have started in both cases after I used Warp Stabilizer on VFR media. (Don't quote me on that...)

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Jeff M commented
I figured out why this was happening. I was saving my project files to a folder that was also syncing to my Google Drive, which caused this error multiple times. I've seen this kind of behavior before with cloud sync services... where one program is trying to modify a file at the same time a cloud service is trying to sync it. The last few days, I've been pausing the sync on Google Drive while I work in Premiere and it hasn't happened again at all.