If you’re not expecting any URL or server address changes, you can force the server URL and disable background auto discover in Outlook. I believe the issue is related to AutoDiscover when having multiple accounts and things get crossed.
The issue will usually happen repeatedly. Outlook for MacOS seems to have a sporadic issue when you add multiple Exchange (Office 365) accounts where both accounts will show “Not Connected” at random times and the only way to force a reconnect is to close Outlook and re-open it.