This seems to be a bug in the slicer. I’m seeing the same issue in OrcaSlicer if the skirt height is set to more than one layer (even if the skirt is disabled). This makes sense if a skirt is used (can’t print a continuous spiral if you have to switch between printing the object and the skirt), but not if it’s disabled.

What slicer are you using? If it’s a PrusaSlicer fork, this is likely the same bug.

Tightening the belts seems to have helped, but I will have to do some more printing to be sure. Thanks for your help!

The belts are parallel to the axis, but I will try tightening them some more.

This is a 40x40 cube printed in vase mode:

The corners look pretty okay on that:

But I don’t see these kind of results on real-world parts. I guess I have to print some more test parts to narrow down the problem.

The part is just pretty small, I have the EW set to 0.45 mm withe 0.4 nozzle. But I will try turning it down further.

The rounding looks much more extreme than what I would expect or have seen on other printers I worked with (mostly Ultimakers).

You can try to disable software End-Stops with this gcode:

M211 S0

Be aware though, if you do that, there is nothing preventing your printer from trying to move beyond the hardware limits and possibly destroying itself. You can enable the end-stops again with:

M211 S1

Marlin reference page