This occured today when I was trying to check a file in manually. Any thoughts?
Most likely the future version of SOLIDWORKS drawing file
What’s so weird is that I’m saving that with sw 2021. I’m on PDM 2021. This used to work.
What specific workflow are you using? I have a 2021 test vault and client so I can see if I can replicate that.
@Gklein got this one right. Turned out to be a broken serial number field that is linked to an add-in.
I do what I can 
it helps when you have access to the SW Knowledgebase.