I've made no such configuration change. My server is as stock as it can be without any modifications needed for XF. This server runs nothing but 6 XF sites. I'm sure some day someone will figure out this issue. My belief remains is that it is because I don't have a Google Tag Manager setup. If you google the 404 error you'll see the common resolution relates to GTM.
It's definitely not a GTM issue... it's
any request to that route on your web server. The way XenForo works is it handles the route. For example there is no threads directory in the root of your XenForo install, the request is passed along to XenForo to handle how it sees fit.
An example of this is if you go here:
"random" isn't a directory on your web server and the whole request is passed along to XF. XF doesn't have that route, so it returns a 404 page not found error (but formatted by XF).
In the case of just the javascript route:
See how the error is different? It's nothing to do with tag manager, rather your web server (is choosing to not pass the request along to XenForo for whatever reason), and you end up with the generic 404 not found from the web server (not XenForo) because the request never makes it to XenForo.
Because of that (and how you can see the request never gets to XenForo like the first URL does), I would bet you any amount of money it's a web server config issue. However, I'm not trying to take your money and it's also neither here nor there at this point because I changed the route to "jc". Which your web server
is passing along to XenForo (but the route doesn't exist yet because you don't have the the unreleased version installed yet):
If you look at that same route here at XenForo (or try any other random XF site), you won't see the generic web server error, rather the XF error because the request is being passed along to XenForo by the web server: