vc4-400 idex - interpreting shaper graph - ghosting on y-Axis - VFA on x-Axis

Hello, I need help troubleshooting my new vCore4 Idex.

What have I already tested?
- I have replaced all flange bearings with the heavy duty idlers
- checked that all screws are firmly in place
- checked the alignment of the gantry, the y-axis runs smoothly without jerking.
- cleaned and lightly greased all linear carriages. The carriages run cleanly on the rails and have a slight resistance due to the pre-tension.
- x-belts for T0 and T1 are exactly the same length. The gantry is not tilted when the belts are tensioned.
- the y-belts are exactly the same length.
- I have tested various belt tensions of 5.80 - 6.5mm with the biqu tension tool. A good average is 6.00mm. The belt tension feels good compared to a Voron printer. The tighter the belts are, the louder the printer becomes and the bigger the peaks in the graphs are.

the x graph for t0 and t1 are very similar, but there are 2 peaks. a large one at around 52hz, which is okay. but there is also a slight peak at around 35hz.
the y graph of t0 has 3 peaks. a large one, again at 52hz and two medium ones at around 35 and 60hz.
the y graph of t1 is completely broken, which I don't understand.

when I compare x of t0 and t1, I see very similar graphs for different print heads. A different motor and a different belt are moving.
the y axis is the same for both print heads, though. all belts and all motors move together - why are the graphs so extremely different?

When I create the belt graphs, x left and x right are exactly on top of each other, which looks very good. But here too, y is very different. Does this graph even work for idex?

The print image looks like this:
- Straight lines on y are printed cleanly and without VFA. Movements of the x-axis do not cause ghosting. It looks fine.
- Straight lines on the x-axis have strong VFA at 90-110mm/s, from 130mm/s no VFA can be seen. The y-axis creates ghosting that the ei shaper cannot compensate for.

I would be happy if someone could help me find the error.