Sign in window popping up all the time disappearing for just a fraction of a second. 

  1. Regedit:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity, create a DWORD value named EnableADAL and set it to zero. Under the same registry key, create a DWORD value named DisableADALatopWAMOverride and set it to 1

2) Reboot PC

3) Open outlook, it will open prompt this time but will not accept credentials > delete entries you created  in 1)

You don´t want to keep these entries, trust me.

Exit regedit and reboot