The reset did the trick, thanks. No idea why that would've been necessary though, since I have not changed or added any presets at all, ever. Seems to me more likely an Adobe bug from the upgrade installation. Oh well. I'm going significantly less crazy now, so thanks again for the pointer.
If I have a rectangle Live Shape with round corners and use the direct selection tool to select the anchors point to one size only, when I move them using the shift key, the entire rectangle is moved, instead of only the side I had selected. Same issue, if I use the arrows keys on computer to fine-tune the positions of it.