When you see error message like this
Warning: Illegal string offset ‘ID’ in /var/www/example.com/library/Mpdf/mpdf.php on line 13394
There is a way to pinpoint the problem at first glance. Experienced programmer sees it in such way:
Warning: Illegal string offset ‘ID’ in /var/www/wiktor.szkolenia/library/Mpdf/mpdf.php on line 13394
Here’s your real problem.