Frustum / Immersive Display Pro / P3Dv3 not working

[pixelcrash]

Dear community,

I am helping a friend to set up his P3D flight simulator with three projectors using
Immersive Display Pro and Immersive Display Pro Calibration.

The screen is covering 170° (-85° to 80°) and round (height: 105 cm).
We marked a grid off 24 columns and 11 rows on the
screen and calibrated it with IDP-Calibration. When doing the frustum with the tunnel image
and any other calibration image - everything is perfectly fine. The edge blending is perfect, and the lines are all straight.

I exported all the .calib files (single and multi) and the viewgroup. I overwrite the viewgroud in
ProgramData/LookheadMartin/... and update the IDs for the displays.

When starting P3D, everything is fine except, that the middle projector has an offset (down).
When flying the edge blending is visible, and the image is "rolling".

What could I have done wrong?
What I did so far

1. I measured our project wall, and the radius is not perfect. Could this be a problem?

2. I moved the height position of the frustum in IDP-Calibration - recalculated it and exported everything again.
The middle projector is now sometimes higher or lower depends on the position. What is the correct frustum setting for a Boing 737?

I am already trying to solve this for two days - any hints and help is highly welcome

Georg Kettele