Hi,
If a user has the Manage shortlist permission, then the referrer is stored when loading a report page, and that referrer is then used after the inspector form is submitted, to redirect back to. I think this is so we can try and send someone back to the page they came from - whether that's the /around page, their shortlist page, an all reports page, or so on, but I'm not sure it works in all situations.
In this case, my guess is the municipality made a normal update (which does a POST to /report/update). They then filled in the inspector form and submitted that, and then the auto-redirect after that presumably redirected back to /report/update (as a GET, as that was the referrer) which then returns the 404 as you say (as it's only used for POSTing update forms). I can't see any other way this could happen, anyway. I guess we could try and not use the referrer if the previous page was a POST, which could help with this precise situation, if it is that.