This just happened to me yesterday (using SW2018 SP2.) As some of you know, you try to save the SW part file and it gives you this error dialog "Fail to save document". While I was in this state (file in session but cannot save my recent changes to it), I read most of the forum posts on this topic and tried everything that I could: "Save-as" copy, Pack-n'-Go, save as to different directory, removed addin's etc.
I turned of my Windows7 indexing because I have been having issues with it - apart from SW. In the end, SW won, and I had to kill my session of SolidWorks. and restart. As a result I lost about 3 hours worth of work. BTW, the same file opened, does now save, without this issue... at least for now.
I'm opening this thread to see if anyone else has had success in overcoming this issue recently. This is a bad one, because, first, I cannot reproduce it for and SR-SPR and second, it isn't like something you can submit to your VAR, because when your in this situation, it is all or nothing, if you know what I mean.