Hi This project. It works at my side. Can you CTRL+F5 to refresh the editor and try again? 

Hi UserSupport, After registering this issue, I have continued to work on the project.  I also tried to find a way to enable the layers again.  While I was playing with the layer settings window, suddenly it worked again. I tried to reproduce the issue, but without luck.  In the mean time, I have saved my project so often, that the version that had the issue can no longer be restored. If you're able to restore my version of 5 days ago, you'll probably see the issue.  The issue remained after closing the project, logging out, logging in again.  It seemed that the problem was stored with the project.  At the time I had that problem, it didn't appear on any of my other projects. I'll try some more to reproduce it.  If any luck, I'll let you know. Regards, Christoph