(SW or HW)Serial communication impossible between Yun and SeeedstudioGPRS shield

Hi all,

we have an Arduino Yun laying here, with which it is impossible to get communication between it and a seeedstudio GPRS shield. We've tried both via software serial as well as hardware serial. In the latter case, we are aware that it might interfere with the bridge between the 2 processors on the Yun.

The sketch we're using can be found at the following wiki link from the seeedstudio's website under step 1:

Because the Yun shares similarities with the Arduino Leonardo, we understand that pins 8 through 16 can only be used.

So, the pin configuration is now as follows:
10 + 11 on Yun go to 0 (rx) and 1 (tx) on the GPRS Shield.

When we configure the shield to use hardware serial communication, and check Serial.available(), we only get '63' as response, which might have something to do with the 64byte limit of the Yun's memory, correct?
When we configure the shield to use software serial communication, and check Serial.available(), the only thing that gets returned is "0".

We've already tried the altSerial library (which uses pins 5 and 13 on the Arduino for communication), but unfortunately without any luck.

What could be the problem? What else can we try to make the shield work properly with the Yun?