v2.11.3 All Channel Forms returning blank page on submit

Submitting any channel form results in a blank page, no error messages of any kind regardless of error display settings, nothing in the PHP or MySQL logs. For testing purposes, I created a new channel with no custom fields and a template with only an unstyled, barebones channel form and got the same result. Tried as multiple different users with no luck. Including jquery and JS assets and not. Basically, we’ve tried every combination of options that we could and get exactly the same result each time. This doesn’t happen with the comment form, only the channel form. Oh, and it happens on localhost as well as on test servers.

Looking at the browser console, the request is generated and sent successfully, but there’s no response data. It’s making the connection successfully, in other words, but then nothing is returned. It’s almost as if whatever routing is supposed to happen when a POST is made never happens, and there’s no fallback response or error handling.

Does this ring a bell with anyone? We’ve been banging our heads against this for a while now and we’re at our collective wit’s end.