Obligation to quote an unpublished source if I am the source? Let’s say that, during my undergraduate studies, I have written an essay on some subject. It was a seminar paper, which never got published anywhere.

Now, while writing my master’s thesis, I find that one line of thought I’d like to express is pretty much exactly what I had written in that seminar paper.

Would I have to formally quote and cite that (unpublished) seminar paper (which I had written myself), or can I just repeat what I have written there without marking it as quote?

It feels awkward quoting myself, but I don’t want to run afoul of any misconduct here.
Avoiding self-plagiarism is very much different from avoiding plagiarism. In the latter case your goal is to avoid using someone's else's work without credits. In the first case you avoid publishing the same piece of work twice (say, avoid inflating your cv). Thus avoiding self-plagiarism means avoiding reusing only your published work without references. So theoretically if you were using your unpublished essay in a new work, there would be no need to cite it. However there are several things to consider. First, as you're writing a master thesis, and not a journal paper, this is a qualifying work rather than just research, and you're are supposed to invest a certain amount of work in this assignment. In this sense reusing an earlier work may be considered sparing some efforts you're supposed to invest to get your degree. Hence, you may want to indicate that this piece of work was done earlier and should not count towards rating your work on the thesis.
Second, as your essay was written within the framework of some assignment given by a professor/instructor, you should acknowledge their input when reusing the essay. Did that person suggest the topic, guide your work or otherwise contribute to the essay? You should clearly give credits to their role, and in this sense it would also be better to mention that you're relying on your earlier essay written in the class of professor XYZ.