[Kivi]

OK, to answer several questions. First, here is the video with new chain.
Sound is not very good because i blocked the mic on the phone a little, but I trimmed the video to be short and to contain the problematic noisy, slappy, jumpy chain part.

Here are some photos of other components:
1pic: sprocket wit retainer removed
2pic: sprocket out side
3pic: sprocket inside
4pic: counter shaft. Looks OK? There is about 1mm of radial play and no axial play on the shaft.
5pic: rear sprocket
I think the rear sprocket is not that worn. There are marks on the sprocket indicating that it was used with worn (prolonged) chain. It is not a problem to replace the rear sprocket if I remove this problem first.

And for the end, the video that explains why I suspect for sprocket missalignment. Let me explain. With the retainer plate removed, when I spin the wheel forward, front sprocket moves to the left for about 2-3mm. When I spin it backward, f. sprocket moves to the right for same distance. I believe, when there is no retainer on front sprocket, the front sprocket "automatically" alligns to the rear sprocket to the ideal position (this may be dependent on chain condition etc).



I have no tools to check sprocket/chain alignment. This last video pushes me to think about frame/swing arm bendind. I have a suspicion that rear wheel axle (thus also maybe swing arm axle) is not perpendicular to the bike axis. My eyes are maybe fooling me, but I think that, when I look the bike from the rear, and when the bike is level, the rear wheel top is moved a little to the left. Thus, rear sprocket top end also would be tilted to the left, and this maybe is driving the front sprocket to the left.
Maybe the frame bolts for the swingarm are not drilled very precise, thus tilting whole swing arm a little out of bike axis. I know from my own expiriance that the frame on this bike is poorly made, not that much of poor quality material, but poorly in terms on precision. I have a spare swingarm identical to mine, which I might try replacing, to try to straighten the rear axle...