RDP Connection with different Port / Port redirection

We've tried to connect Server with different RDP port than 3389(security by obscurity )

Entering
Servername:12345 as Destination or
129.168.1.1:12345 as IP adress (both is working with mstsc.exe )
results in
"Connection failed, the computer name contains illegal characters".

How can we use RDP on Server with such port redirection?
Didn't find anything in Forum!?
Is this so abnormal?
Could you realise it with next patch (Using ASG-RD 2015 (v8.0.5091.0)?

Thanks
Harke