@VolkerS 

Thank you for your post.
Could you please check whether the camera is still showing as online/connected at the moment?
Have you tried refreshing the page multiple times? Does the live view re-appear after a refresh?
You might also test with a different browser to rule out any client-side issues.

Lastly, have you attempted to manage the camera through VMS? VIGI VMS
I recommend using VMS for device management—it often provides a more stable interface.