I was just about to register the support case when the same thing happened to one of the other gateways, this time I knew exactly what happened…
I had just changed the https port on the devices. This turned out to be the problem all along. Once I realized the issue, I checked the authentication script for Node-Red and realized that it uses the https-interface to authenticate the user.
Maybe this could help someone else in the future… 
Thank you for your support!