Your browser does not seem to support JavaScript. As a result, your viewing experience will be diminished, and you have been placed in read-only mode.
Please download a browser that supports JavaScript, or enable it if it's disabled (i.e. NoScript).
Today it works again. It is really an intermitent issue.
Problem solved by trying to join an existing game, which updated some game files. After that, I can download mods without issues.