Just a quick follow up on this: I bought some new batteries and they arrived yesterday, and now I have a cat! It seems to have been an issue with the batteries, but with the new ones the calibration went smoothly and I got the cat to sit and stand and wave. Walking is a bit of a problem still, will look into that. Thanks for your help and recommendations!

Thank you for the quick and good explanation. I am using two batteries which are brand new, so they should have more than 3.5V. I have adjusted the potentiometer but it's very difficult to adjust it once it has been mounted to the frame. It's also recommended in the assembly instructions that the potentiometer be left at the lowest value, which I guess would lead to this issue? It would be nice if the instructions would mention when and how to adjust the potentiometer to the correct value. One question: why can the servos rotate all the way around? I'm not an expert on servos, but the ones that I have used before are not able to do that, which would prevent this kind of problem, I think?