Hi Joost,
You broke te continuity of both E9 and GR 120-Sentier du Littoral.

In this changeset and in the following changesets : 
https://www.openstreetmap.org/changeset/173755893
https://www.openstreetmap.org/changeset/173757206

You could see discontinuities with WMT  (press analyse route) : 
https://hiking.waymarkedtrails.org/#route?id=2139379&type=relation&map=8.0/50.5787/1.976

Could you fix the route (better with JOSM) or do you need help ?
Do you speak french ?

Best regards,
Yves — Groupe OSM Plein Air