Thanks for all hints.
So far I tried:
- I played with T2_TWI and T4_TWI in USI_TWI_Master.h. I changed the default values of 5/4 (1MHz) to 0.625/0.5 (divided by 8 for 8MHz) and 0.3/0.42 (divided by 16.5 for 16.5MHz).
- I tried with TinyWireM.write (instead of TinyWireM.send).
- I tried with DrAzzy's ATTinyCore AVR-Wire-library - but gave up in an early state because of so many compiler errors.
... unfortunately without success 
Regards
deetee