Olimexino 32u4: Wire.endTransmission() hangs
I've did not got it working with Wire. Hence I've implemented a I2C protocol myself using bit-banging and then it works.

Update from 2017-September-16

The problem was not Wire.h but a specialty of the Olimexino 32u4 board. I've only found out by using a Real Time Clock connected to the UEXT port's I2C bus, that I have to switch on the power for the UEXT port by setting D8 to LOW. Otherwise SDA or SCL seem to be pulled down preventing any I2C communication.