I hope I am not hijacking this thread.
I am getting the tcpdf error as well. My steps. I jsut ran an upgrade from 2.0.1 - 2.3.2+
I added a certification activity to the end of a course using pretty much defaults stuff. it worked.
I then went to the admin area and uplaoded a new image. I went back to the cert activity I had embedded. I chose the new image (where are they stored anyway? the new images) and hit save and view. I viewed the certificate and the image worked. I then took the A4_embedded type folder, copied it and renamed it to test. I then uploaded that directory tot he types directory. I then went to the course certificate activitya nd edited it to use the new type.
I got this (with debugging on)
Strict Standards: Redefining already defined constructor for class questionnaire inC:\inetpub\moodle_sandbox\mod\questionnaire\locallib.php on line 94
Strict Standards: Redefining already defined constructor for class questionnaire_question inC:\inetpub\moodle_sandbox\mod\questionnaire\questiontypes\questiontypes.class.php on line 196
ï»¿TCPDF ERROR: Some data has already been output, can't send PDF file
Without debugging I get the TCPDF ERROR. I then swapped it back to the A4_embeded type and it worked.
LEt me try using the non embedded certificate.php