Wobbly XY surface

[rlogiacco]

I'm aware of the wobbly ZY plane caused by the vertical rods not being straight, but I'm experiencing a very irregular XY plane on big prints and I wonder what might be causing this.

https://imgur.com/a/F1aSLGR

The effect is greatly visible on the left side of the printer, but seems worse on the back side of the plate (Y carriage toward the front) so I'm wondering if I have a bent frame or something lose...