I have tried to log in from an other PC and edit. Same error.
I created a new account and tried to edit. Same error.
I tried to switch from Swedish to English. Same error.
The error comes only when trying to use the Visual editor. The text mode editor is ok.
I also see “Error contacting the Parsoid/RESTBase server (HTTP 404)” with the Visual editor (which I never use, but that’s another story).
Edit: I’ve mentioned on #osm-dev on IRC. The logs there suggested some sort of change containing some of the words from your error message** on Tuesday PM.