Aweful Graphs .... regardless what I'm trying :-(

Dear all, I can't get my VC4 400 IDEX running. I disassambled the whole gantry 3 times, all mgn rails are running absolutely smooth, Y is moving like a charm, .... and in the end, when I put everything togehter ....🤮 checked all screws over and over again, tried different belt tensions .... always more/less the same result. My VC3.1 has a perfect grapgh, but this IDEX drives me crazy. Any help/idea would be so helpful - thank you so much!