You are in Germany, take advantage of the Kaufland. It may be that she doesn't like the bottle you are using. I use the Nuk bottles with the wide based nipple. It's the only bottle my 2 month old will take. We've started giving her pumped breast milk with Bauchwohl Tee mixed. The Bauchwohl Tee is for newborns and up. If the formula is giving her gas, like my love of brocoli is doing to my baby, then you might want to mix in some of the tea. It has worked wonders.
So, my advice is to check out the bottle and how she takes the bottle with just plain breast milk. If it's with some dificulty you may need to switch over. (The American style bottles didn't work well for either of my kids.) If she's gassy, then try out the Bauchwohl Tee, it works so much better than the gas drops you can buy at the BX and there's no fear of an overdose. Lastly, you may be trying to switch her at the wrong time... for instance, if she's begining to have teething pains or her teeth are already comming in. If she's trying to get her teeth early I'd try brushing her gums with a finger brush first and then offer the warmed bottle. And you can always try one of HIPP's formulas rather than the ones they sell on Post. A lot of German baby products that I've tried tend to be a bit more gentle and often are more logical, so maybe the formula is as well.
Best of Luck to you :)