Receiving gift from student before grading MSc thesis. Is it ethical?

In my opinion you are asking yourself the wrong question. The important problem is not if it feels like a bribe to you, but if it could look like a bribe to others.

Let's say you have another student whose thesis you are grading at the same time and he ends up with a worse grade. This might have been an objective decision, but still, if this other student now hears about the gift you got, he will start to wonder if maybe giving you a nice gift would have gotten him the same grade.

Since the gift is quite small, you are most likely legally in the clear, however in my opinion, ethically it wasn't the right thing to do. It is probably to late to return the gift right now, but for similar future situations, it is probably better to decline gifts until the grade is set in stone. Not only from an ethical point of view but also since it will give you an aura of fairness and incorruptibility at the low cost of a few cheap beers.

Most students know that you grade according to the marking scheme and they offer the gift unconditionally.

It is the "other" students who you have to be careful of, and you get a "feeling" about those - a rare case usually.

Edit based on comment: There are policies dealing with this in some institutions, where, if the value of the gift is below a given amount then it is fine and accepted by all parties including management. However, if the value is above that limit then it must be declared and the line manager or line manager +1 makes the decision as to whether the gift can be kept or it has to be returned or held by the department etc.

No, this is not ethical; don't accept such gifts.

Whether it is ethical to receive gifts from students before grading their MSc thesis

It is not. A gift influences the receiver in favor of the giver. It also carries an implicit indication of the assumption the receiver will give a high, pleasing grade; you wouldn't gift someone who stuck it to you in grading and made you appear incompetent, would you?

(or any other project where students feel like they need to thank you for).

This is the more serious problem. The gratitude an M.Sc. candidate feels towards his thesis graders/readers/examiners is not the kind that should translate - in practice and in their minds - to personal gifts.

I would try to reflect upon why that is that such gifting seems like a reasonable thing to try and do - to students, and apparently to you. I would then think of how to educate students and faculty that this kind of gratitude should be expressed by, say, contributing to the faculy/department; volunteering for some academia-related cause etc. - something roundabout and diffuse in a larger community of people.

Lately I have encountered this situation where a student gave me a present before I was able to fully grade his work.

You're implying that it matters very much whether it's before or after. This is part of the work you are (hopefully) paid to do, and part of your duties towards the larger academic community. You're not doing a favor here.

The gift was rather small, just a few bottles of good quality beers which are not that expensive where I'm from.

So, multiple bottles of somewhat-expensive alcoholic beverage. Totally inappropriate, even if you're in a beer-drinking culture.

He also said: "you can drink this when grading my work, ha ha". We got along quite well, so at that time I didn't feel like it was some sort of bribe (which I still don't feel like it was).

You should have told him: "I appreciate the sentiment, but you know, there is at least the appearance of an ethical issue here, so please don't be offended, but I have to decline. I suggest you bring the beer to your thesis presentation" [assuming you have that custom, where people give a seminar-style talk about their thesis] / "farewell party."