"Ask a Question" page uses the 404 template

This is quite an odd one, our “Ask a Question” page uses the 404 template.

It still works as normal, but the <title> is “Document not found” and the body’s class is error404.

This doesn’t happen on our local test server, so I assume it is one of the multiple network-activated plugins running on the live site.

Any suggestions to find out how/why?

This happened on a clean install of Q&A and persisted through all the customisation I have heaped upon it.