Editing document in 2021 is slow and "Save as" causes "Unknown error"

I have a document created in 2012. It is not very complicated.

But when I try to edit it in AI 2021 it becomes painfully slow. And if I try to "save as" it causes an "unknown error"

Same if I try saving to PDF or EPS.

If I open the same file which I also saved to Acrobat in 2012, and "save as" from Acrobat, it saves OK. But when I open any page of the newly saved PDF file in AI 2021 it still has the same problem.

I tried resetting prefs and it didn't help.

I have AI 2019 installed and when I opened the file there it worked and was editable and saves as anything without a problem. But if I open the file I had just (successfully) saved (in AI 2019) in AI 2021, it still has the same issues as before!