behind the light, behind the interior trim, there should be a square plastic box bolt. Remove this then pull the light backwards towards you. Its head in by that bolt + x2 pressure clips on the outside edge.
No known issues that I am aware of. All it takes is someone giving you a tiny tap in a supermarket carpark or something in order to create a point of ingress. Water "creeps" unfortunately, so the crack can be microscopic. Personally I would look for a replacement unit. Throwing this at a dealership will prolly generate the "cluster is damaged / cracked and not under warranty" standard response.
Same issue in my LED rear outer right with my local dealer & Audi UK not at all interested, more here with a write up. Grabbed a used mint replacement for £50 from eBay & took me 5 mins to swap it over. Inspected my unit when removed & no visible damage at all, left it to dry out at room temp & now in my spares stock.