Thanks for all the suggestions, but I was just want to point out that my issue is not ssl related, I am not connecting via https, and the issue is reproducible across different browsers from Chrome to Edge to Safari and Opera.
Firmware 380.69, Chrome Version 65.0.3325.162 on Win 10 (issue reproducible on Android and iPad).
I have major problems connecting to the admin page; the login prompt appears, I log in and get a half-rendered page:
The Chrome dev console reveals a lot of files failed to load:
If I click on...