Checking the battery is a good idea. Strange things can happen when the voltage drops too much. Another thing to consider is adjusting the brake light switch instead of replacing it. The ESP control module looks at the brake light switch as well as the membrane travel sensor in the brake booster. It compares the two signals for plausability. Apparently, there's a timing issue between the two signals that can be resolved by shimming the (self-adjusting) brake light switch when it's installed. Simply depress the brake pedal and pull the plunger on the switch out to the limit of it's travel. Before releasing the brake pedal (which will adjust the switch), attach a 1.5mm spacer to the tab on the pedal that contacts the plunger. In the workshop we usually use a wire-tie, but I've seen guys use all sorts of things. With the spacer in place, release the pedal. This will cause the switch to adjust one extra "click". I've used this method many times to fix a few stubborn cars, with good results. Remember to remove the spacer when you're done. Let us know what happens.