Harley Davidson chain belt running jolty
Okay, it seems I indeed made a mistake during building, which I solved afterwards:

When assembling the rear mudguard, I noticed that the rear wheel had quite a bit more variability in its sideways adjustment and was quite a bit off center, specifically too far to the left. This caused the chain belt to either run slightly skewed or have friction on some part of the frame. When I adjusted the wheel more towards the center, it ran a lot smoother.

However, this was only part of the problem. The actual mistake I only realized at the end when seeing a little yellow half-stud wide technic bush, which I missed in instruction step 25 already, where it was placed under the gear on which the chain runs and is supposed to space it further away from the frame. Fortunately I was able to repair that without dismantling the entire thing.