Maybe that's caused by some incompatibility between the STK-menu (SIM tool kit) of the adaptor and the one of the congstar SIM. Have you tried changing the order of SIMs? Have you tried the adaptor with the congstar SIM in another handset?
Btw there's a relatively dualSIM handset from Samsung, the "C5212 DuoS", which is sold for
€ 120 in Germany (unfortunately supports only GSM900/1800). Maybe that's a working alternative for you.