It seems to be an issue with the router at the Slingbox location. It should not work without the port forwarding on it. Did you create a DMZ rule for the Slingbox before?
Take a look at the following links. You'll find the most common reasons for this behavior.
thanks for your answer which unfortunately is not quite correct.
The slingbox does work fine without portforwarding enabled.
Usually the slingbox uses TCP when portforwarding enabled, however if no port is opened on the router, it switches to SNATT which I believe is a combination of UDP and TCP.... When the slingplayer uses UDP it does not acknowledge every packet, TCP does this.
Since my box is located in Germany and I live in the US there is a big latency whenever I ping the box which causes the slingbox not to stream correctly when portforwarding (TCP) is enabled. If I close the port it uses SNATT/UDP which works absolutely perfect.
I found this answer here, these people really know what they are talking about