I had a Jungheinrich EFGDFAC20 w hitch had a similar problem in that it went through about 6 encoder bearings, some lasted 2 days others 2-3 weeks, genuine SKF bearings and no one including Jungheinrich had a lue why! The only thing I came up with out of desperation was that something was spiking them and knocking them out, I fitted a chain on the bottom of the chassis which dragged on the floor to discharge static and since then not a problem!
We have lots of these that don't have a static chain and have never had an encoder go for some unknown reason, it was just this particular truck. I have heard of static problems when trucks are fitted with non marking tyres but this one was fitted with standard blacks. Just a thought!