I would suggest making a full backup of the osTicket site files, make a full backup of the osTicket database (just in case), move all the osTicket site files outside the HTML directory, download latest version of osTicket, unzip the download, move all the files from the
upload/ directory from the unzipped archive to the HTML directory, copy over your
include/ost-config.php file from the site files backup to the HTML directory, copy over all plugins and language packs from the site files backup, upgrade if required, and retest.
Sometimes people just copy/replace files and it doesn't copy over all files, some overwrites fail or get half re-written, etc. It's best to do the steps above each time you upgrade so no old files are lingering or prevent new files from being copied, etc.