Thanks CW2, you were spot on! :-)
The netduino plus 2 is faster than the old one and so I never noticed that I ignored the max write cycle time - it just wasn't able to poll the eeprom fast enough.
Thanks Chris, this could have been the problem - in the meantime I updated to the newest release so I am not absolutely sure if swapping the reference or updating to the newest firmware solved my problems.
Anyway, both problems are fixed - thanks for the support!
Take care,
Martin