Symptoms

During Registration wizard (connected with the K2 Service account), Administrative OAuth Token part:
A new screen is displayed for login, we put the login of the K2 Service account but get the following error message:
"Sorry, but we're having trouble signing you in. we received a bad request."
 

Diagnoses

The connected user (in this case K2 Service) need to be "Global Administrator" to do the registration wizard.

 

Resolution

Setup the connected user (We advise to be the K2 Service account) to be the "Global Administrator" : Go to AZURE AD (Admin center), "Users and groups", "All users", search for your user, Directory Role and select "Global Administrator"