Roundcube not loading email contents

Here's another quick one! I had a Roundcube instance, chugging along fine.

Then one day it just stopped loading emails! Everything else was all fine - no problems with login etc, but trying to preview an email just game me a sadfaec. Chrome couldn't load the contents, and told me that the server had refused to connect:

So of course, I can't just let this go - being able to see the contents of the emails is relatively important! But what the heck was causing it?

I spent some time in Chrome's developer tools and console, as well as time grepping and tailing log files... this would have all been fixed much earlier if I were running Firefox I think, as it's a bit more explicit with errors.

So, what was the fix? It was nothing to do with Roundcube, routing, firewalls, or reverse proxies. (Seriously, I checked all of these...)

The problem is that, last week, I'd modified the HTTP Headers on the reverse proxy to include:

   X-Frame-Options ("DENY")

... no good. It all came right when I changed the X-Frame-Options directive to "SAMEORIGIN". Suddenly, all fixed.

Sheesh!