Outlook 2016 Security Window Keeps Popping Up    

Only fix that has worked for us so far is rolling back to previous version of Outlook 

Here it is if anyone else needs:

At the command prompt, run the following commands:

cd %programfiles%Common FilesMicrosoft SharedClickToRun

officec2rclient.exe /update user updatetoversion=16.0.8326.2107

Don’t forget to disable updates until Microsoft gets around to fixing this.

Outlook – File – office account – office updates – update options – disable updates

Thanks to @Sean Varley &  http://www.brucebnews.com/2017/08/outlook-and-the-never-ending-password-prompt/