@Hana
Download the code from GitHub develop-next branch, backup your current osTicket directory files to a separate/different directory to make the current osTicket directory completely empty, put the new code files from GitHub in the current osTicket directory, copy over your config file and any plugins and language packs, restart Apache, restart PHP-FPM (if you’re running it), kill all sessions in the database by truncating the ost_session table and retest.
Cheers.