update 10.11 to headless macmini failes?

Hello

We have an macmini 10.9.5 server which we want to update to 10.11

This Server we manage with ScreenSharing and Apple Remote Desktop. We started the Update and after about 45 min the macmini was again visible with Remote Desktop, but we have no access anymore. The color of the icon is "deep blue" so everything should be ok

See the pictures enclosed

In the last years I updated more as 40 headless server with older OS X Systems and never had a problem. This is the first Update to 10.11. Very frustrated that this based is not working.

I am able to copy the server.app to the server, but the screen will not be availible :-((

Is their a way to access this macmini, to finish the Update and install the new server.app? Which services blocks the access with Remote Desktop

Regards

Gérard