During this winter, will it be alright to wear a shirt-jeans and a jacket for the PhD interview at Max Plack Institute?
(Physicist here, experience with PhD admissions at British and German universities, including one Max Planck institute). In my field, I have never come across an actual dress code for PhD interviews. Nonetheless, if you dress too casually, especially older faculty members might get the impression that you are not taking it seriously enough. On the other hand, with suit & tie some people might perceive you as overdressed and maybe a bit stuffy. So being somewhere in the middle, e.g. jeans, shirt & jacket as you suggest, is a good approach.
I strongly assume that it is similar for other STEM fields (which account for most of the Max Planck institutes), but it can be completely different in other fields that tend to value formal attire (e.g. law).