| | Problem solved by Provider!
The aforementioned problem was suddenly solved likely by the provider setting some flags. It started working without my changing anything in the scripts from one day to the next. Therefore now GPRS connection to the internet through a 8100 without using a Blackberry BIS/BES or data plan works fine. The carrier is generic (Eastern Europe) and the device is unlocked. The SIM is prepaid and has a data "plan" per month.