Can't open Excel 2019 password protected files after updating to Excel 2021

I have several Excel 2019 password protected workbooks that, after updating to Excel 2021 Professional Plus, I can't open. Not password protected files are fine.
When I try to open the protected file I get the following error:
We have found a problem with the content of Xxxxx.xIsx'.
Do you want us to try to retrieve as much content as possible?
If you trust the source of this workbook, click Yes.

If I press Yes, I get the following error:
Microsoft Excel cannot open or repair this encrypted file because it is corrupted

It strikes me as odd that all my password protected files are corrupt, but the non protected files (that are in the same location) are fine.

I seem to rememeber an article about this problem from CodeProject, but can't locate it.

Any ideas?