In the new macOS X.15 beta 4 release it loads and shows all of the folders, both on remote accounts and on the iMac, but it stalls loading new messages and sending new messages. It also does not quit correctly as the app indicates that it isn't responding. Going to the Admin acct with the same mail accounts, things work fine. But of course the Admin account doesn't have the same On My Mac folders, but an older version that has not been updated. I use no add-on features.
Any suggestions about how to fix? Maybe forcing mail to rebuild the database of folders and mail? How do you do that?