AdminTodos: "Unexpected response entity" error on Firefox 57 (Quantum)

Accessing the server-manager from Firefox 57.0 (Quantum) 64-bit on GNU/Linux shows the html source code of admintodo message (or so I think) inside a modal window, titled Unexpected resopnse entity!

I tested it also starting firefox in safe mode (add-ons disabled…) and clearing cache. No errors on firefox console. If someone can verify it…

Starting with a new user profile it works as expected.
Problem was a custom websites.referrersEnabled property.