Mac mail message import failed: Make sure you have available space in your home folder and try again.

I have had a problem with Mac Mail for about a year, where 1) it won't automatically open the Message Viewer whenever I open it or return to it and 2) Mail's Search randomly stops working and claims my search term is in every single email, or in zero of them.

In trying to correct this problem, I found a suggestion to duplicate the account using Time Machine Restore. This worked well and most of my files and settings seemed to come across, except Mail. Mail kept giving me the error message "mail message import failed: Make sure you have available space in your home folder and try again." Apparently this isn't really the problem and I found a message about it here in the community, but the info needs to be updated for Big Sur. To fix this issue, I went to the Library\Mail\v8 folder, did GetInfo, unlocked the padlock, clicked the circle with 3 dots, and clicked "Apply to enclosed items", then relocked. The next time I opened Mail, it imported everything just fine.

This fixed the Mac Mail import issue and I am hoping the Message Viewer and Search issues too.