Y linear rails offset or profile twisted

Hello. I have had my V-Core 3.1 Drucker for 3 years. The two linear rails for the Y axis have never run really smoothly since they were first installed. Now I reassembled the printer for the third time new and here's what I saw. If I only attach the 2020 profile to the right joiner plate and move back and forth, the profile remains slightly below the left joiner plate, but is always at the same height. But if I mount it to the left joiner plate, the height on the opposite side changes when moving. You can see it quite well in the pictures and the video. There is also a gap between the plate and the profile. I built the frame twice following the RatRig instructions. The third time, I started at the top with lying upside down with the profiles and added and aligned all the others one after the other. The left profile would therefore have to be a little lower, or the right one profile a little higher. But aren't the dimensions specified by the profiles? Am I making a mistake in my thinking?