How to understand the lectures on the fly as much as possible? My problem is, that during the lectures my mind just goes blank and I cannot follow for a few seconds. Consequently I will have trouble catching up with what was just explained. This repeats more and more as the time goes on. 

So the information I am hearing just turns into white-noise as the lecture proceeds. Obviously this is something that other people also have to struggle with. But I feel (there is no other way to tell) that this issue is more hindering to me than to others.

Maybe you others have your ways to cope with that. I am curious. 

All the best
One thing that helps me is taking pictures of the board periodically throughout the lecture. This allows me to spend less time frantically copying notes as to not miss anything, and spend more time actually listening and processing the information. Also, I would assume that your professor doesn't mind questions during lecture. If something catches you off guard and pulls you out of it (your brain might just be stopping to see if things are making sense), I would just raise your hand and ask the professor to re-explain it. Most of the time, there are other students in the class who are hung up on the same exact point.
Understanding on the fly is possible when you already know intimately what is being lectured. This intimate knowledge comes after you have spent time and effort in something called study: a process whereby a pre-concept (a new notion of which one can have some hypotheses about their meaning thanks to intuition coming from prior knowledge) tranforms into a concept through an activity of check of its coungrence with respect to all concepts already owned by the learner. So my suggestion is not to attend lectures unless they are given by authorities, and in this case do it after having studied.