Believe it or not, that mostly sounds like good news to me. You now know where the rattle is.

Quote Originally Posted by Alexej View Post
...as to the light test...I used a metal ruler, which seems to be very straight...so, it starts with a glimpse of light at the 22nd fret...then 17th fret...then 15th - 9th...and then between the 5th and 3rd fret till the end/nut...So, those seem to be waves on the fretboard...I can imagine, that this would not be an issue with frets, but on a fretless, this seems unplayable to me...
That looks like the problem to me too. Very clear in the pics. My neck was similar near the net, FWIW. You might never know if it were fretted, since mostly you would only need to worry about the tops of the frets being level...but to play properly a fretless needs to be flatter, particularly if you want a nice action.

...I tried to make the neck as flat as possible, but what really makes me nervous, that after the 5th fret the fretboard/neck makes a curve and has about 1 mm difference at the nut, compared to the 5th fret...So in my opinion I would have to sand the whole fretboard 1 mm down, from 22nd till the 5th or 3rd fret...to kind of get a flat surface...
That sounds horrible in my mind ...

...For the sanding, I am not sure how thick the fretboard is...with all this information, is it possible to solve the problem at all?
It should be possible. You would do it exactly the way you say. I don't think you have to worry too much about the thickness of the fretboard. Typically the fretboard is around 6mm. I am sure that I sanded away at least 1mm of fretboard in making mine fretless. In my case it was only partly to get it flat. I had some chipping around the slot from pulling the frets. I had to sand those out. I have had no bad effects. Your neck is otherwise pretty flat, so I am guessing that you will only need a very minor adjustment to the truss rod.

Also, what choice do you have? If you are nervous about it, you can work it down slowly with a fine-grit sandpaper. In my case it never occurred to me that fretboard thickness might be an issue, so I went at it with a pretty coarse grit (100, I think). When it started to get close, I worked down to 220. When it looked good with 220, I finish sanded with 320, and then a maroon 3M sanding pad, finishing with a gray 3M pad. If you do it that way it doesn't take that long. You'll want to wear a mask though, since it will produces a lot of dust.

And I noticed some traces on the fretboard, I think it comes from the strings...hope it can be seen in the picture...That is not normal, is it? I mean I barely "played" it
I just looked at some of your earlier pics and see that you are using roundwounds with it. Some people actually use roundwound strings to cut the grooves in their nut. That gives you an idea how aggressive they will be on your fretboard. I use flatwounds on mine. (Fender 9050L Stainless Steel Flatwound Long Scale Light Bass Strings). These are similar to the strings that I used on my upright bass back in the day. The don't seem to be marking the fretboard...at least not yet. I see recommendations for tapewounds, but I don't trust them. First, they are generally just tape over roundwounds. It's just a guess, but that suggests to me they might be more aggressive than flatwounds. Also, I have had exactly two sets of tapewounds for my fretted basses, and they were not durable. The tape broke on both sets after a few months, and the string was totally useless.

So, even after some builds...still did not check every detail before starting the build :/
Welcome to the club!