Does the IKEA Besta series back panel lack stability?

Hello maybe someone can help. I bought an Ikea Besta cabinet (60x40x128cm). Assembly was easy, doors mounted and shelves inserted. When adjusting the doors I noticed that the carcass warps to the left. The carcass is aligned straight and not mounted to the wall. The doors are on the right side. I have to press the top of the carcass slightly to the right because it pulls to the left. About 2.5 cm I would say. In my opinion it’s the back panel because it does not provide the necessary stability. This becomes clear when you remove the top panel. The side panels are warped to the left. Does anyone have an idea without maybe mounting the cabinet to the wall?

There are 2 options: 1. Remove the back panel, align everything at a right angle, and put the back panel on accordingly. 2. If it still seems wobbly, you can give the piece of furniture the desired stability with a diagonally stretched strap, which you staple in place while aligned.

There are 2 options: 1. Remove the back panel, align everything at right angles, and put the back panel back on accordingly. 2. If it still appears wobbly, you can give the piece of furniture the desired stability with a diagonally stretched strap, which you staple in the aligned state.

The thing is: the back panel is not fastened but only inserted into rails. I already had the back panel out – it didn’t help.

Then the solution with the strap applies - provided the shelf can be forced into position with the installed back panel.

Then the solution with the strap applies - provided the shelf can be forced into position with the back panel installed.

What kind of strap would you use?

The thing is: the back panel is not fastened but only slid into rails. I already had the back panel removed - it didn't help

Weren't there any nails included to nail the back panel to the sides, if the back panel remains movable in the grooves, it will never be stable