Mail cannot access your mailbox information because it’s already in use
Mail.app has started popping up this error on a 2017 iMac 21 fusion drive Monterey 12.6.1
"Mail cannot access your mailbox information because it’s already in use"
Restarting Mail makes it go away for awhile and the iCloud account will send and recieve. Googling finds a few short unproductive discussions of this exact same error message over the last 14 years. This screenshot of the full error adds to the mystery.
Anyone had this problem? And fixed it? Or at least understood it?
iMac Line (2012 and Later)