Is X: a network drive?
I have had this happen to me before and have had success with recovering the corrupted file by doing the following steps.
Attempt the following:
Open SOLIDWORKS Rx.
Launch SolidWorks in one of the two Safe Modes. The second option that bypasses the tools/option settings has been what works for corrupted files I have come across.
If you are able to open and save the corrupted model with no errors, close the current session of SWX safe mode and re launch SWX as normal. Test that the model in question opens and saves without errors.
If opening SWX with either of these two safe mode options does not allow you to save the corrupted model, contact your VAR.
Citing original thread and post by John Stoltzfus.
Joe, thank you for that tip, it did the trick. It seems that the Rx did change a couple minor settings, but everything is working.
From my understanding, your original file is not corrupt, the one that is opened is. Are you opening the files in lightweight? Normally when a file is corrupted you can't open it, if you can open it and it doesn't save then it means that something in your workflow might have caused corruption in the process.
I've had a few users have this message pop-up when trying to save a lightweight assembly, which is why I asked. When the assembly is opened normally, it didn't give the error.