I ran into an issue that exists only for Sitecore 9.1 initial release. If you have used your maximum licensed number of users, you will be presented with a dialog that allows you to either add more users temporarily, or kick off a user so you can take her seat.
However, clicking Add Users or Kick Off User will just refresh the screen, and you are still not able to log in to Sitecore. Very frustrating!
I opened a support case with Sitecore and they provided this hotfix for the issue: http://dl.sitecore.net/hotfix/SC%20Hotfix%20308084-1.zip. This has solved the problem for us and we can now boost or kick users again.
NOTE: This hotfix is ONLY for Sitecore 9.1 initial release.
Hope this is helpful for someone out there.