Live Preview does not work anymore

For some days I cannot use the live view to make changes in the theme. I have not installed any new Plug-Ins. What did I already try?

– reinstall Wordpess
– reinstall Oxygen
– deactivated all Plug-Ins

Same result. When I try to open the live view I get the navigation of the theme configuration on the left frame. In the right frame I always get the whole dashboard again. But I never get the live view in the right frame.

Is there an idea in the universum?

Regards
Erwin