I can confirm that the problem for the given stream also exists when running the script from home
Yes, I do get your error when running your script testSLClient.py here.
But running slclient.py directly does not seem to give any errors or lost packets:
python obspy/seedlink/slclient.py -S GE_SLIT:HHZ geofon.gfz-potsdam.de:18000 -p
So it seems that there may be something going wrong with your sub-classing of slclient ???