Ohlins fork cap

Had that on mine from new on one leg if I understand the problem correctly . The cap comes apart and you will see that the preload adjuster is held in by a large nut with rubber seal underneath to take up any play and prevent water ingress. On mine the nut was maxed out but there was still play ( seal was fine) so I added a very thin washer. It was actually a crankshaft thrust washer.   Ohlins sell the cap in parts inner/outer etc so you would need to decide what parts you need if you want to go the replacement route.  Could be the nut has just worked loose on yours or the rubber o ring has disintegrated after all this time. You need a deep socket to reach the nut. 

I would try an Ohlins dealer like Kais