Plesk osTicket version 1.12 is not compatible with PHP 8. This works with lower version of PHPs.
Either you lower the PHP version or upgrade the osTicket that works with PHP 8.
If the osTicket version is upgraded on Plesk, please let this forum know how the latest osTicket was upgraded on Plesk for others to get updated.
You will get below error for version mismatch ---
"The app requires PHP version later than 5.4 but earlier than 7.1. Your PHP is 8.0.17. If PHP is not available, ensure that the PHP support is selected for the domain."
Thanks,