Did you also alter the size (sidewall depth or hieght I guess) so that the end result was the same outside diameter as the old set up? Larger rims with lower profile tires.
I know traction control/active handeling are programmed with the stock sizes so if you put 18's all around it thinks the rears are slipping and causes problems.
I see you kept the difference (two sizes) but think the overall diameter may be your problem. The wheel speed sensor just knows how may times it turning, not how big it is, so you would think you would be ok, but I''d look at the outside diameter as a start.