Just curious … have you rebooted at all at any time since the upgrade? If you have not, DON’T until you have secured an off-site backup of your data … this has gone too well so far - good luck always runs out at some point.
Once you have an offsite backup of your /home/user-data/backup/encrypted directory and your secret key file you can try to reboot if you desire. If that will change anything - I do not know. The proposed fix is to remove the email account in the admin area, by clicking on ‘archive account’ on the Mail>Users page and then to add the account back again on the same page. If that user is an admin account be sure that it is not the only admin account first.
Please note that due to things that are way beyond my knowledge level things MAY fail. If that happens you will need to reinstall everything from scratch. I expect that the chance of this happening is very low, but want you to be prepared for the worst case scenario.