Today, I finally checked the headset. I could indeed hear some grinding noise and so I adjusted the headset.
I did some test rides: When breaking abruptly and hard, the break reacts well and there is no stuttering.
But when breaking with a bit more feeling and less power, the stuttering is still there (see attached)
I adjusted the break once more but it didn't make any difference.

Any other suggestions?

@PLA: Could you please explain why to change the wheel?

Thanks, I will definitely double-check.
My rides felt normal otherwise but the brake power has quite some leverage through the fork, right?
Did you "feel" anything while riding?

Thanks, @fiveup89. I will double-check this. I am pretty sure I did it before because I was wondering if they are the same or not but did not think about the washer.

Thanks also for the Oz Cycle video. Good inspiration where to get fitting o-rings

Different topic: I was fiddling with the SRAM Rival front derailleur and even though there setup tool is a great help, I have issues keeping the derailleur aligned with the chainring when tightening the screw. Any advice or shim-recommendations or Oz-videos how to manage this?

Thank you, dinorello.
What video is that? Do you have a link for me?

I am not 100% sure about the headset, might need to chat with Wendy once more It's pretty tight with SRAM hoses so I was considering using a bit of grease to make sure things are moving when putting everything together.

Thanks for the advice re foam tubing. Actually used exactly the product you are recommending 

I can confirm the weights. I also have a 54 cm super light and (again) kitchen scale says:
Frame incl. rear hanger: 926.5 g
Fork (uncut): 406.9 g
Handlebar: 329.4 g
Seatpost: 164.9 g

Here is my work-in-progress. Frame, handlebar and wheels are from LC.
For the decal, I simply provided the font and there is a little graphic on the headtube where I used simple online tools to reduce a picture to black and white and then made the background transparent.

I am very happy with the build quality so far.
And the communication with LC during the whole process was really, really great!

Thank you for your thoughts.

My bike fitter recommended me the 400mm width and I feel comfy with it. I got an older bike which has a 380mm handlebar but I mainly use in on the trainer or when my other bike is in the garage; anyway, the smaller width is OK but I notice that my elbows tend to "latch in" while I stay more relaxed on the wider bar.

I live in a rather flat, wavy at the best, environment but with lots of wind (the sea is not too far away). I usually do longer rides (4-5 hours) and I hardly ever participate in competitions.
I like riding in the drops especially when fighting the wind but I tend to switch often because there is traffic, traffic lights or crossings where I need full control. I guess, in the end, I am riding most of the time on the hoods.

@PLA: Nothing in particular, yet. Read some articles and reviews.

@jonathanf2, I don't have the "knee hitting bars" issue but your thoughts on stability are helpful. Usually, I go low against the wind but stay in the saddle; I don't feel very comfy sprinting or accelerating on the drops.
So, I might stick with my 400 on the hoods and go slightly wider on the drops!?

Once more regarding the T47 BB: I think I started with a wrong assumption and would like to double-check with you...
My assumption was that the frame is 86.5 and that BB cups + BB sleeves has a fix width (86.5 or 85.5 e.g.).

But looking at the Hambini Guide (thx by the way) and a number of products and specifications, I understand that the BB sleeve is normally only fix on one cup-side and can travel (a bit) in the other cup, right?

So, we are talking about the width after installation. And if I install a SRAM BB and end up with 86.5 width, I need to correct this width by using 2 rather than 3 mm spacers on the drive side to achieve the correct chain line, correct?