In a thesis, which comes first - Abstract or Acknowledgements? I am writing my masters thesis now. I saw some theses with the acknowledgement chapter after the abstract, in other cases it's the other way around.

I think the acknowledgement chapter is not related to the scientific work in the thesis, and thus should not be put between the abstract and the introduction. An abstract could be nice in the beginning if we're looking at a paper for example, but for a thesis (usually >80 pages) the reader would have to turn the pages anyway.
You should check the guidelines from your university. I'd assume that they would have requirements for the order. If not, I always prefer the acknowledgements should come first, that way the non-scientific stuff is out of the way.