I have been trying unsuccessfully to take a copy of your site for testing. Duplicator won't complete the scan of your site, and while I was able to make an archive with All in One WP Migration it failed to complete when importing it to a test site.
I didn't run any checks on your site because it looks like a live site, do you have a staging site with the same issue you can use for testing?
Can you switch theme to twentyseventeen and see if the error still shows.
If so, disable all non-Toolset plugins and see if the error still appears. If not then you can identify which plugin the conflict occurs with by re-activating them in batches and re-testing in a process of elimination.
If there don't appear to be any conflicts I would need a SQL database dump as an alternative way of reproducing our site, which you could upload to dropbox or similar and share here.