New warning code in header

Hi, I got an unusual problem with a long warning code in the header at my webpage; www.rikakvinnor.se 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
Elisabeth

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:
https://wordpress.org/support/article/debugging-in-wordpress/

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

Kind Regards, Roman.
aThemes Support