Thx for reply. Yes, it was sealed so it's definitely brand new. There was a tape over keys as well. Every stuff in original package. They've had also B stock item (wonder why it was returned, maybe because the same issue) but I've intentionally not bought that just to prevent such issues I think I really got a lemon. Maybe it was a bad batch but then Casio quality control sucks badly. I can't imagine all Casio keyboards are like this. I really hope not. So also adjacent keys does not sound good to you ? There are bit noisy as well in my opinion but at least I can feel dampening and I could live with that. But this one E4 is really crazy, it's like you said maybe there is no dampening at all. I will try to make other video of all keys and you may judge. There is also other keyboard (I think A7, not sure) which is similar to noisy E4 but far not so bad.
Yeah, I think it would be fixable and I saw you pictures - nice work, but this is brand new product with 3 year warranty and I don't want to void it. I will write Thomann what's their opinion and hopefully they will agree it's a fault and send a courier for replacement.