Ref T13662. I ran into this while trying to reproduce the mention issue discussed there.
Currently, the root document (with slug "/") attempts to preview using the URI /phriction/preview// (with two // at the end). This is collapsed into "/phriction/preview/" by Apache if "MergeSlashes On" is configured, which is the default behavior. The route then 404s.
Instead, just use "/phriction/preview/?slug=/" so this endpoint functions properly regardless of the "MergeSlashes" configuration.