Office 365 Hybrid Constant Credential Prompts
When an Exchange users is migrated to Office 365 they may get constant credential prompts. Even if the user enters the correct credentials the prompt will come back in a few minutes. The user might also lose the ability to access shared mailboxes and public folders.
The fix is to set all of the reg keys below to 1, doing so will enable modern authentication.
All versions 2013+ Create this even if you are on 2016 where it is enabled by default
Restart Outlook and the prompts should go away.