unable to open password protected files

We performed a clean install of Catalina today due to a slowdown in performance on the iMac

All went well and the Mac is now as good as it ever was.

The only problem is we can't open password protected files on the iMac for some reason.

We created the files on the iMac last year and have successfully opened these files until today.

Non protected files open without issue.

We are not entering the wrong password as the same file opens on our MacBook Pro running Monterey.

Any advice to sort the problem?