Basic certificate not showing any text in the pdf

Hi.

When I finish a course and click the certificate link I see the pdf with the image I have chosen, but there is no text.
I assume the following text that is in the settings has to be there:
FIRST_NAME LAST_NAME
has successfully completed the course
and so on ...

I tried this on another wp installation with all plugins turned off and the problem is still there. Am I doing something wrong?