Cannot merge a compare. Getting 500 response on initial AJAX request

When trying to merge a compare I am getting a 500 response on the initial AJAX request in console. The page loads and appears to be working, the merge button does nothing and I am not able to use the WebGUI to merge.

This happens on Mac Safari, Chrome, and FF.
In Ubuntu 18: in Chrome, Chromium, and FF.

The response returns this:

#!json

{"type": "error", "error": {"message": "Something went wrong", "id": "b8e23814f29548d587c7d608c7a7164c"}}