Outlook password pops up and disappears quickly

Try Logging out of all office products:

Open Outlook to see if the prompt pops up. If Not go to next step:

NEXT STEP:

Try removing the Microsoft Account and restart Outlook.

GOTO:

Settings>>Accounts>>Access work or school

Delete work or school account

Press the Windows key + I and select Accounts in the left pane.

Select the Access work or school option.

Now, click the Disconnect button in front of the affected account.

Kon Belieu