Many thanks for this post !
I did not have the courage to search for a solution to this (given the quite generic error message), and stopped using JOSM for a while (actually contributing at all).

It's really amazing that a thing breaking so much can reach a popular branch, and remain for days.

Because they don't know OSM exists.
Or maybe they think its quality is way below that of GoogleMaps, and can't imagine it will ever top it.

But that will change over time, hopefully. A little advertising and polish on the tools, and the masses will fall for OSM !