System Administrator receives an "Insufficient Privileges" error when launching an OAuth button while logged in as a user (even though the user does not receive the error).
This is due to a Salesforce security update that does not allow System Administrators to log in as a user and launch an OAuth button.
What to Do
A workaround is to add a session ID to the button only when you need to login as the user. We recommend removing the session ID once you are done.
Button with session ID:
Button without session ID: