I have been wondering at which point the solving turns into guessing, for example if you're using chain strategies and eliminate numbers by that. I always feel like this is also kind of guessing. When do you think is the border between guessing and using a strategy?

I think this topic's a little subjective. There's certain techniques that come easier to some solvers than others. I agree with Thomas' basic explanation here - http://www.gmpuzzles.com/blog/2013/03/ask-dr-sudoku-12-the-line-mus... (coincidentally posted around the same time as this query). In that explanation he's given the most practical reasoning. Anything a solver can see in his mind without having to put some digits in to reach a contradiction can be called logic. Its just that different solvers may not be able to see the same amount of steps/information as other solvers.

Whether a puzzle is logically solvable and whether a specific person can solve it logically are two different things IMO. As a solver, many times I've had some logic pointed out to me that I missed even after hours of staring. Similarly, as an author I've sometimes had to point out to solvers where/what the logic is to my puzzles too.

Thank you very much. I hadn't seen this post yet. It helped me very much, although I think it's not that easy to solve the puzzle without guesswork if you're imagination isn't the best and you constantly forget which number you picked to try out.