Apologies. I meant to include those in my original post. There are no related entries that I can find in the apache logs. However, in the php_errors.log I see this a handful of times:

[18-Sep-2019 18:55:20 UTC] PHP Fatal error: Maximum execution time of 30 seconds exceeded in /var/www/osticket/include/mpdf/mpdf.php on line31108`

If I view the mpdf.php file, this is the function in question, line 31108 is the first while:

function purify_utf8($html,$lo=true) {
// For HTML
// Checks string is valid UTF-8 encoded
// converts html_entities > ASCII 127 to UTF-8
// Only exception - leaves low ASCII entities e.g. < & etc.
// Leaves in particular < to distinguish from tag marker
if (!$this->is_utf8($html)) {
echo "<p><b>HTML contains invalid UTF-8 character(s)</b></p>";
while (mb_convert_encoding(mb_convert_encoding($html, "UTF-32", "UTF-8"), "UTF-8", "UTF-32") != $html) {
$a = iconv('UTF-8', 'UTF-8', $html);
echo ($a);
$pos = $start = strlen($a);
$err = '';
while ( ord(substr($html,$pos,1)) > 128 ) {
$err .= '[[#'.ord(substr($html,$pos,1)).']]';
$pos++;
}
echo '<span style="color:red; font-weight:bold">'.$err.'</span>';
$html = substr($html, $pos);
}
echo $html;
$this->Error("");
}
$html = preg_replace("/\r/", "", $html );

Tried updating max_execution_time, no change to the behavior.

Can you confirm what version of mpdf should be installed with 1.12.1?

I now have it reporting a proper error that will be more helpful:

[Tue Oct 01 14:17:08.149843 2019] [php7:error] [pid 16268] [client 192.168.0.221:53588] PHP Fatal error: Uncaught ArgumentCountError: Too few arguments to function mb_internal_encoding(), 0 passed in /var/www/osticket/include/mpdf/vendor/mpdf/mpdf/src/Mpdf.php on line 1096 and exactly 1 expected in /var/www/osticket/bootstrap.php:283\nStack trace:\n#0 /var/www/osticket/include/mpdf/vendor/mpdf/mpdf/src/Mpdf.php(1096): mb_internal_encoding()\n#1 /var/www/osticket/include/class.pdf.php(70): Mpdf\\Mpdf->__construct(Array)\n#2 /var/www/osticket/include/class.ticket.php(3119): Ticket2PDF->__construct(Object(Ticket), 'Letter', '0')\n#3 /var/www/osticket/scp/tickets.php(498): Ticket->pdfExport('Letter', '0')\n#4 {main}\n thrown in /var/www/osticket/bootstrap.php on line 283, referer: http://192.168.0.250/scp/tickets.php?id=129941

I found the solution. Searching out similar problems I found that:

/var/www/osticket/include/mpdf/vendor/mpdf/mpdf/src/Mpdf.php on line 1096

was related to the php extension mbstring. Installing mbstring string fixed the problem. So it would seem that that is now a required extension going forward.