Can't connect to Minecraft Server via Telekom Speedport

I have problems to connect to a certain (Nitrado hosted) Minecraft Server. When I used my FritzBox Router, it worked perfectly. But now I use Telekom Speedport and I got the [Server Connector #5/ERROR]: Couldn't connect to server io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection refused: no further information message. I already restarted my Router, Minecraft and my PC without success. I am not sure what else I should do, since other Servers are working perfectly.