Never seen any post on any forum that has that fault. But, the first thing on the A4 that comes to when any one says anything about faults after a rain is the drain until the ECU box. If it is clogged, there are electrical relays that will sit in water causing varied faults. The ECU box is under the cowl cover to the right of the battery. I would start there. If I remember correctly, the drain can be seen below the black box and to the right. The other drains are in the sun roof area and if clogged, water can intrude in your cabin. See link below far a good explanation on the drains.