I finally managed to send this work e-mail I was so reluctant to send (Basically: I know you worked hard for 2 month on this but it's not helpful and we discussed before why it wouldn't be and it's no suprise that it isn't) Sometimes it takes time to find the right tone, but the right tone can be very important especially if you potentially have to work together again.
My boss needs to have that same conversation with that persons boss and he told me today "haven't talked to him yet... I'm waiting to get into the right mood" :D I feel you!
@windmills My university department is really easy-going overall, the head of department is a really nice, young guy and overall everyone is just really really nice ;) I'm not sure if german work places are generally stricter than others. I would maybe expect people being ambitious and "angry" with others if they are not.