How should personal/health be presented when explaining a bad grade at an interview?
Don't dwell on it, and don't bring it up unless your interviewer specifically asks about it. A couple "pass" grades doesn't sound too terrible to me, lots of people have much more serious marks on their academic record, such as a semester in which they failed most/all of their classes. Even in that case, though, I wouldn't bring it up in an interview.

Your academic record will be viewed as a whole, rather than in microscopic detail. A concerning pattern would be poor grades in courses central to the work you want to do, like underperforming in math compared to other subjects if you are in a STEM field. Two "pass" grades in one semester is not such a pattern.

If an interviewer asks about it (I actually doubt they will, or if they do, it might actually be more tongue-in-cheek than serious: more of a comment on the rest of your strong record), I wouldn't dwell on it, either. You don't really owe them any details. "Yeah, I had a stressful family situation that semester and didn't feel as focused on my coursework" is more than enough (could also mention that it was specific to the time of the exams that solely determined the grades).

The rest of your record speaks for itself, so let it do that. Keep your focus on what's actually important in the interview, which is your interest in research. I think an interviewer would find it strange, maybe uncomfortable, and counterproductive if you spent the interview trying to explain two grades rather than talking about research. Once you start a research career, your past grades become mostly meaningless.

(as a disclaimer, I'm from the US and don't have any experience interviewing in Europe, though I'm fairly confident my answer translates or I wouldn't have written it)