Microsoft Outlook prompts for a password

If Microsoft Outlook keeps prompting for credentials on every start-up you might find that it tries to connect the server in a wrong way.

First try to run Microsoft Outlook in safe mode. 

outlook.exe /safe

If no prompt for credentials appears - try to disable all your add-ons and restart outlook.

If it does not help - do the following:

Find this screen in the settings of your account

Make sure that "Connect to Microsoft Exchange using HTTP" is unchecked and restart Outlook again. That was the step that helped me out.