Levo Gen 3 Slip under load

[Titaniumdean]

Hey a problem I had before has reoccurred , I have a slip in the gears under heavy load followed by a large pinging sound coming from the cassette. Here in Germany you lose the bike for about three weeks for repairs so put it in and turned out to be the cassette loose on the hub, bikes done 2000km on original gearing, all good, done about 100km and it’s started again, they said chain and cassette were fine, does this sound like a rear hub issue and is it commonplace? Was considering doing it myself but heard it’s complex?
Thanks in advance