The first JS error is generally caused by what I said above.
I haven’t got a clue what the second thing you posted is. Looks like a list of files and functions that have been run but I don’t see any errors in that.
If you have thousands of posts then yes, the AJAX request could be timing out. There have been other reports of this. But again, it could depend on other things that are running. However, if one relationship field loads then they all should since each one will create a separate ajax request. Usually this happens when trying to search, not when loading the page.
https://support.advancedcustomfields.com/new-ticket/
