We recently had a few tickets come in with Outlook trying to connect to Exchange error. Outlook on their mobile device worked as did the web. The problem only occurred on their desktop. The first time it happened, nothing showed up in event viewer so we disabled the antivirus. This did not fix the problem. As it turns out, the Outlook sign-in box was coming up but very quickly and then disappearing. It was happening so quickly that it was barely noticeable. This gave us a lead on what the problem might be. After some research, we found this solution:

1. Log in as local user having the problem
2. Close all Office Apps
3. Place these keys in the registry:

[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity] "DisableADALatopWAMOverride"=dword:00000001 [HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity] "DisableAADWAM"=dword:00000001   Exit the registry which will save your changes and then try logging in again. If you are in a domain environment, it would be best to deploy this change via group policy.   If this does not fix the issue for you, drop us a line as we may be able to help.