This started to happen a while back on a secondary pc of mine after upgrading to Firefox Quantum.
I wasn’t sure there was a relation, but I also didn’t mind much, because on my main pc there was no problem (still using a pre-Quantum FF version)
But I now upgraded my main PC to FF Quantum, and the same is happening.
After clearing the browser cache I can connect again, but a next time I will get this error message again:
I couldn’t find other reports on this, but I can hardly imagine I am the only one experiencing this?