Sequoia 15.01 creating Excel file saving issues and corrupting workbooks

Help. For a number of years now I have used a WDMyCloudMirror as my NAS and have had no issues at all. Now I am consistently getting a problem that only seems to be related to the saving of Excel Workbooks.

I recently updated my MacOS to Sequoia 15.01 from when this problem seems to have started from. Firmware for the WSMyCloud (5.29.102) was updated back in September and is the latest.

When I open a workbook from the WDMyCloud all is OK. Then when I click to close it by clicking on the red button and follow the option to save I get the following message:

When I try and open the file I get the following:

The file appears to be corrupted and cannot be opened (will not open in Numbers either).

This only happens when I click on the red close option and follow the Excel option to save or not. If I save first then close by clicking on the red button all appears OK.

If I copy any "good" workbook to my iMac first, opening, saving, closing all behaves absolutely as it should with no issues.

I also have a MacBook running BigSur 11.7.10. When I open, save, close etc. from the MacBook I do not have any issues either.

Has anyone else had the same problem or know what the problem and resolution is?