Gianluca
IIS is deprecating PHP 8 and not providing any support for it. So I wouldn't rely on IIS too much longer.
1.) It appears your instance of MySQL is older than 5.5 which cannot handle FULL TEXT for ost__search table. If you upgrade MySQL to 5.5+ this error will go away.
2.) You probably had other issues that is causing this issue. Please review all 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.
Cheers.