iMac Mail with Gmail account
iMac with Sonoma 14.5 now constantly denying loading of Gmail account saying "rebuilding or cancel" due to gmail being used elsewhere! Never a problem prior to 14.5.
iMac 21.5″, macOS 14.5
Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?
Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.
Quit Mail & login to your WebMail in a Browser, does that work OK?
Open Mail again.
If you have no POP accounts & they’re all IMAP accounts then in Mail’s Mailbox menu choose Rebuild.