the mystery of the wonky corners

[Jonathan]

To pan in the toolpath preview it's either middle click and drag, or right click and drag...I don't remember which it is.

If the acelleration is far too low and you're using G64 (constant speed) mode, then your corners will all get rounded... but it would have to be very low for that to happen. 2500mm/s^2 is pretty high (sure that's not the rapid feedrate). You could try putting G61 at the beginning of the code to force the machine to stop at each co-ordinate precisely.

Where you've tried to cut the slot, parallel to Y, it does look like one of the X motors has lost position and skewed the gantry, causing the subsequent cut to not be parallel to the first. Are the pulleys definitely tight and not slipping on the shafts?

Can you post or send me the code so I can check it?