Ralf,
If I were working on this problem in the lab, then I would check to make sure that the I2C signals have the proper pull-up resistors on them. We normally pull them up to 3.3V through a 10 kohm resistor. It looks lie your voltage is okay, but I would check the resistance. If that is okay, then I would try using a known good setup. I would try using the EV2300 with your bq34z100 module and the MSP430 with a TI bq34z100EVM. The bq34z100 will hold the SCLK signal low if it needs time before it can receive more data. The MSP430 may not be handling this properly.
You should also make sure that the device is not in ROM mode. It will NACK under this condition as well.
Regards
Tom