Please see the screenshots here:
https://ibb.co/MDFbRyM
https://ibb.co/p4Df7MN
The error disappeared after deactivating everything, but appeared as soon as I activate a Divi Child Theme I am using. The error doesn’t seem to have any negative consequences, so it’s not pressing (just thought I’d let you know). If you want to analyze it, I am happy to give you access to a staging copy of the site.
I’m not technically versed enough to provide the full stack trace, sorry. :/
If you deactivate Query Monitor do you still see a fatal error message in that position? You might need to enable WP_DEBUG_DISPLAY for it to show up.
If so, you’ll need to report the problem to the author of your theme.
When I disable Query Monitor and set WP_DEBUG_DISPLAY to true, the following is displayed:
https://i.imgur.com/rRtPUAo.png
By the way, I found that there were some more details in the original error message:
https://i.imgur.com/QYW3BB0.png
https://i.imgur.com/Nrseo7b.png