Update for the record:
I checked the footer.php and head.php and both were containing the lines that the error was about. I found out that it was cause by an add-on (Test Head Footer by Matt Martz) to a plugin (WordPress Pop Up Plugin). I disabled the add-on and now there’s no error message, which seems to be a mistake as I found both lines in the files that the add-on was complaining about.
Big thanks