If the last accessed Host group in "Monitoring -> latest data" no longer exists then an error 500 will be returned and the user account has to be deleted and re-created to regain access to this page. Note that this does NOT happen if only one host in a host group is deleted. When just a single host is deleted a user gets the expected result of a reset search page. This result happened in multiple browsers and multiple computers, confirming it was an account related problem.