VC400 bow in heightmap

Attached is the current heightmap of my V-Core 3.1 400 (Enclosure 2.0) - I've invested around 3-4h of adjusting the extrusions. I've watched the video for finetuning the frame (https://docs.ratrig.com/commissioning-guides/v-core-3-1#build-plate-preparation) and did so accordingly. As far as I can see, the bed is bent down, which points to a bow in the gantry - so I readjusted the linear rails on x-axis and for good measure on y-axis, too. They are tightened at around 1Nm max to avoid bending the extrusion. But to no avail -.- Anyone got another idea? // edit: Using the first layer calibration file from the docs, after having done z-offset etc., rams the nozzle into the bed front right while doing the priming blob. That' really curious.