Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
This seems to have fixed it for me. Just speculating, but I think it is related to the change from Office 365 and what they may have done to the certificates when they renewed. I have a main mailbox and four shared mailboxes on the app formerly known as "Office 365" and I only had to change my main account to solve the problem.
PS: Thank you, Jeff Willette!