How to defend badly graded master thesis?
While I wish I could be more positive, the fundamental fact that is in the German grading system, a grade of 3,7 on a master's thesis is a disastrous grade that is essentially unheard of. Virtually all master's theses receive grades somewhere between 1,0 and 1,7. A grade of 3,7 is very much a "pity" grade that allows you to pass the master's program, but is essentially going to be an albatross around your neck. You will not be able to use the degree to pursue advanced study anywhere in Germany, and gaining employment with that kind of grade will be a lot more difficult.

You will want to check the Prüfungsordnungen (examination rules) for your program to determine if you are allowed to resubmit or repeat the master's thesis, rather than take a grade that low on the thesis. In that case, you may find that option preferable to having to deal with the consequences of a D on the thesis.