> In JOSM you'll need to force a complete Validator run – make sure no objects are selected, when doing so, or it won't work.

Thanks for this information, I didn't know that the JOSM validator gives different results depending if objects are selected or not...

> Or have a look at Geofabrik's OSM Inspector [...] for a visual analysis of the errors.

The error is visible here: http://tools.geofabrik.de/osmi/?view=areas&lon=7.39858&lat=46.94597&zoom=18