IDEX offset of T1 on the far side from VAOC

🤷‍♂️Other

My issue is the prints coming out of my T1 is a kind of rotated regarding the prints from T0. Close to VAOC they're perfectly fine, but on the other side of the bed there is half a millimeter offset. What I did:
- Checked the rear plates with the motors (unscrewed and screwed again, to be sure they're flush to the profile).
- Adjusted the anchoring of the belts to the heads in order to make the front tensioners extend the same amount. It should make the length of both belts pretty much equal.
- Shaper graphs are good. Very low vibration percentage (under 2 for all). Y belt above 5700 MZV, X belts over 7500.
- The frame is quite well assembled. I've measured all diagonals and they have way less then 1 mm difference.
- The Y rails are parallel (as far as I could be precise with a ruler)
- The height map of my VCore 4 400 is with 0.080 mm tolerance when heat soaked, which also confirms the frame is good.
- The gantry is flush to the stoppers in end position.
- The diagonals of both 350 mm printed squares (in two colors) are equal. The skew value is zero. This also confirms the frame is straight.
No idea what to go for next. I bought an IDEX to be able print two filaments, otherwise I'd stick to my Voron, but one month later still no luck with setting that thing...
Any hints or ideas are welcome.