New warning code in header

Hi, I got an unusual problem with a long warning code in the header at my webpage; do you have any suggestion of how to get rid of this? I have the latest versions of both wordpress and sydney theme.

Thank you in advance!

Best regards

Hello Elisabeth,

It looks like a WP Super Cache plugin issue, did you try to deactivate it? By the way, you might want to disable WP_DEBUG, please check this WordPress Support article for more details:

Please feel free to ask any other questions that you might have.

Kind Regards, Roman.
aThemes Support