HTC Vive and Oculus Rift head tracking does not work


Two weeks ago I tried the “VR Adventure Game” with the HTC Vive and everything worked fine. In a few days I want to use CoSpaces with students to develop a small VR application. So I tested it with the Vive again today and unfortunately the head tracking does not work anymore. I can see the Controller and they are moving correctly but the view does not react to the changes of the headset. I also tested the Oculus Rift, different versions of Firefox (65, 64 and 63) and Chrome. With Chrome I can not even see the environment through the Headset - just a white surface. Does anyone have similar issues?

I also experienced the same thing on our Vive. It was working the week before, but when the students tried it on Wednesday, you could only move forward / backward.