Managed to find out a solution by myself.
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removed the three #define NO_PORTx_PINCHANGES
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removed the #include <PinChangeInt.h>
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changed PCintPort::attachInterrupt(pinInclusion, startInclusionInterrupt, RISING); to
attachInterrupt(1, startInclusionInterrupt, RISING);
Now I am able to use the Seeedstudip GPRS shield pins 7 and 8 with SoftwareSerial.
