jeffgumbel
You will need to check your logs (general server logs, webserver error logs, PHP error logs, MySQL/MariaDB error logs, osTicket System Logs, Browser Console logs, etc.) for any related errors.
Have you updated your plugins to the latest ones?
Can you check the backend of the agents that can’t login? Go to the database, go to the staff table, and look at the value of the backend column.
Cheers.