That is a real bummer. I am sad to say I get to see that one more than a little. I hope this isn't something that you see too often though.
Where are you storing and accessing your CAD files from when you get this error from your saves? I ask because this is something that I have seen for at least the last 10 years and it seems to crop up more when files are stored on a network drive, external drive, flash drive, or in a local folder managed by a cloud storage system (like Dropbox, OneDrive, or Google Drive).
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I get that occasionally, but have always been able to get around it by saving to a different name.
This one can be infuriating!
The error message makes everyone cringe, however with 2016-2017 there is an additional dialog box that comes up and asks if you want to save the changes or discard the changes in the assembly, which I always picked yes - save the changes - then I can kill SW, restart SW or restart the computer and the changes were saved. If you got that same message years ago, it would drain the brain and soul and leave me nervous and twitching, now I'm not nervous anymore, but that awful twitch still sucks............
This one comes close, it does give a little explanation, but then I only added constraints to a couple of parts that were in the assembly when I opened it.
Retrieving data ...