Unfortunately, it’s difficult to discern the root cause from the error message alone. I would suggest taking a backup of your website, understanding how to restore the backup if need be, install the Health Check & Troubleshooting plugin, and disable your plugins and themes until you find the culprit. Once found you can reach out to their support to hopefully resolve the issue.
The file line looks like this:
esc_attr( implode( ' ', $item->classes ) ). From that it would seem that the filtered item classes may not be an array.