TCPDF error

[Philipp]

This is a misconfiguration of your webserver.
You have set PHPs error_reporting level to E_ALL

This causes not only WARNINGs and ERRORs but also NOTICEs to be displayed in the browser.
NOTICEs are not really errors, more like hints for the developer.
This is not recommended for production servers.

You should configure your error eporting to E_ERROR or E_WARNING in php.ini , or configure it to be logged to a file instead of displayed in the browser.
Then this problem should go away.