I’m not sure if this is applicable to your situation but some things that can force the OL LOgin Dialog include:
1) Password protected file even if the password is blank.
2) Selecting the option ‘Prompt for a profile to be used’. I believe it’s possible to enable this even if you have only defined 1 profile.
Hope that helps
YOu might need to check to see if the outlook profile on the system matches the user AD credintials. Normally that will prompt for the user profile if it’ s different.