Apple Remote Desktop 3.8.5 - Can't connect to ARD Server

Hello to you,

we are using ARD 3.8.5 to support out iMac's with ARD Client. There is no problem to connect to it, install pkg's or other software. All functions are ok. We are using OSX El Capitan on all machines.

But i have a big problem to connect to our OSX Server with Apple Remote Desktop Server. When i try to connect the window comes and telling me "connect to ...." and nothing is going on, can't connect???

What is it? I try to find it out, but nothing. Need help or a idea

THX

Chris