"System Error &h80070057" / "Can't find project or Library" / "Excel encountered errors during save"

I have an excel file that was created in Excel 2016

Sometimes when being run in Excel 365 I receive the 3 error messages in the Subject line (see attached PDF)

However, the exact same file is able to be opened, saved, worked on, macros run, ... when back in Excel 2016

At wit's end - assume this must be some kind of Excel bug!

{I can't attach the file, since it is a very proprietary model}

Please help - thank you !