I recently came across a problem in a trial where Dynamics 365 kept opening a new tab at the login screen.
I spotted in the URL that it was trying to redirect to crmdynint.com which is a problem I have blogged about before.
To resolve, go to the settings cog (top right) and click Advanced Settings. Navigate to Administration and open System Settings.
In system settings, Scroll down to “Set custom Help URL” and change “Enable Learning Path” to No