Hard to tell as it doesn’t point to any theme file.
Is the error happening every time you reload your website? If so, I would deactivate your plugins one by one and reload your site to see if it stops happening.
The error does not happen often. The error log file started on May 22, and the error occurred 172 times, but there are thousands of unique visitors each day.
When I look at the pages of the website I see no error.
I don’t think that deactivating the plugins would help, since the error does not occur frequently, but only a few times a day.
Hmm, extra difficult to debug then I’m afraid. I haven’t had any other reports of this (and my error_log files don’t have this error) which leads me to believe it’s one of the plugins you’re using.