Jen - I have taken the temp of my butter just because I am anal like that .
It is about any where from 63-68 depending on how long it sits out before I get to it. At 72 my butter cannot get a dent because my finger will go through it. I take the butter out of the fridge, cut it into about 8 cubes. By the time I get to it about oh probably 20-30 minutes later it is at the temp range I mentioned.
Whatever works for you is what one should do. If I let the butter sit to 72 and very soft, I have to put the bowl in the fridge to cool for a bit. That won't work once imopen my store so cool butter it is.
To 1cakatatime - I believe the problem you had was you did not get good stiff peaks before you added the butter. And you might not have gotten stiff peaks because of the kind of egg whites you used. Some, but not all, pastaurized egg whites will not whip up. At my store it says that in the carton. It is hit or miss if it works, so I use fresh. Being you are in Germany and I am in the states, I will not say definitively this is the issue.
Also if it is humid and you have no AC this could be issue too. Just stick the bowl in the fridge if it doesn't set up and see if that helps. Once I had let it set overnight to get a good set up. But it is summer here and now the AC is on all the time, so I have not had that same problem.
Forgive any typos please. I am on my IPad and it takes longer to correct, so I am skipping then proof reading today.