Hi again,
found the bug: the bus i am using is splitted in two 2 x 25cm long busses which is not the best solution. Sometimes because of layout reasons it must be done this way. In this case two 100 Ohm resistors close to the Master in series with SDA and SCL and pullup's 2 x 10kOhm at both ends of the splittet bus make the bus working fine. I checked the timing with a logic analyser and the quality of the signals with a scope, both ok, no glitches or other dirty effects. Hope this help others having same trouble. I must say best solution is always making the bus from one end to the other end in one direction, i tested this also with 80cm. Adding also two 100 Ohm resitors in series with SDA and SCL close to the master makes the signal quality better also.
Both versions run with 80kHz very fine now.
Regards,
Ralf