Hi all!
Just to help someone that could have the same issue printing ticket.

Running osTicket (v1.15.6) on windows servr, PHP Version 7.4.16, I had http 500 trying to print a pdf.
It seems to be a problem of permission on the temp folder sys_temp_dir but it is not or it is a part of the issue.

After many hour I solved the issue simply taking the include/mpdf folder from one other different installation of OSTicket. I took it from the 1.16.x and now it works.

Note that before I also changed the folder path in the class.pdf.php writing in it the path to the folder I create:
parent::__construct(['mode' => 'utf-8', 'format' => $psize, 'tempDir'=>'mydomain/tmp/osticket-cli/']);

Note 2: ceating a php file to print the php parameters (inside i put only <?php phpinfo(); ?>) the system shows a lot of parameters and the value of the parameter about sys_temp_dir is empty.
trying to create a php file to show the value (inside I put <?php echo sys_get_temp_dir() ?>) I obtain a different results: it shows me a server folder like c\windows\temp.
It does not matter because it seem to be an initial setting and the process o printing override this value through the class.pdf.php

Hope this help some other.