I was having a similar issue with the log-in not always working, and returning users to the log-in page.
It turns out OSTicket doesn't like it if the session ID is a different length to what it expects, or contains non-alpha-numeric characters.
You can of course configure this in your php-fpm.d/php.ini etc, but you can also set (potentially more conveniently depending on your hosting arrangement) in the /include/ost-config.php by adding:
ini_set('session.lazy_write', '0');
ini_set('session.sid_length', '32');
ini_set('session.sid_bits_per_character', '4');
... to the end of the file.
Hope this is helpful to some one.