Releasing the design before actually receiving the boards and testing them turned out to be a mistake. Out of three boards that I got from OSHPark, two have a short between the servo power and ground. The third one seems fine.

So I opened the designs and looked through them carefully trace by trace -- but no short there. I opened the gerber files and looked through that -- no short either. The fact that two boards have a short but the third doesn't, suggests that there is some problem with some traces being too close to each other.

So I took a knife, and started cutting the traces on the board, trying to locate the short. I narrowed it down to the lower right quarter of the board -- specifically, to the ground fill on the bottom, between the servo socket pins.

Oops, seems like routing a "thin" trace between two 0.4mm pads doesn't leave enough room? This is a bit strange, because it always worked for me before -- perhaps I was just lucky.

Anyways, I went through the design and changed all the pads to 0.35mm, and also made some other traces passing through narrow spaces thinner. I also deleted the old designs from OSHPark, and added this new, corrected one. Ordered it, and we will see how that one works.