I had the same problem for my site and also got it for this page here.
It has something to do with Privacy badger, but it can be difficult to reproduce.
For my site, it happened in the following situation:
- Privacy badger enabled, domain of my site was whitelisted explicitly under “Whitelisted domains”.
- When going to the site via a Google search result click.
- Force HTTPS enabled. (Not sure if this matters.)
- Firefox as browser
The problem didn’t occur when going to the site directly, only when going to it via Google Search result click. A refresh on the page didn’t work, unless the browser was restarted.
Also, HTTP request headers didn’t seem to matter (such as removing referrer).
The problem went away when I removed the whitelist of my site from Privacy Badger. But I also got it on this page here, not sure why exactly.
Seems like some kind of a strange bug in Privacy Badger.