Problems starting (Keyless)

My 2011 Audi A8 has an issue when starting. It was working as it's supposed to, foot on the brake, press the button and then Pesto it starts. Then it started playing up. Initially, sometimes when I would put my foot on the brake and press start it would start but I would get an error warning that the start/stop was not working. This tended to happen after there had been rain. Now it is a problem all the time. If I press the brake, then press start once the ignition comes on but it doesn't start. If I press the button a second time (with foot on brake) it starts but with the warning that the start/stop isn't working. If I press it once (foot on or off the brake) then with foot off (if it was on) press it a second time to turn the ignition off, then foot on brake, press the button, it starts and the start/stop is there. Go figure.

Anybody got any ideas? I talked to the garage I use but they had no idea.