I had this issue since the last week, after upgrading to Opera Developer 41.0.2329.0. At the same time, the ImTranslator plugin stopped working.
When I removed it and visited the Addons site to reinstall it, the site looked unstyled, and I couldn't install the addon as well.
I read previously that this might be an issue (and it was!) when you're behind a corporate firewall, which I am.
I connected to the site using my personal hotpot, the site loaded successfully and I was able to install the addon, which now also works fine.
When I connected back to the corporate network, the site still works, probably with the cached css.