Mysterious clutch problem!

Dear members of this website. I really hope you can advise me on the next problem with my vehicle listed above MJ.

Bought last year, converted it to Camper, replaced clutch (garage) last spring. Drove no more than 6,000km since. Borrowed the Scudo to a friend who took it for a long trip through Germany and at one point I get a phone call that the clutch is slipping. Road assistance shows up, tells my friend the clutch is gone and needs replacement. Insurance pays for transport of Scudo back to Switzerland. Scudo arrives back in Switzerland after some weeks (summertime). My garage guy gets in the Scudo and drives off no problem. Calls me that it looks fine. I pick it up, drive it intensively for a couple hours. Uphill, downhill, engine breaking, low speed in higher gears (where it usually shows if a clutch is slipping) but nothing that looks anything like a broken clutch or any other problems. My friend drove for some time on the German Autobahn that day (I was wondering it might be something which only happens when hot) but the problem only showed up after being parked for some hours and then driving off again in a town.

Now to me it usually is quite clear when a clutch is going. Can it be something with the hydraulic clutch cylinder? Any ideas where to look for? Since the problem is not obvious, I am reluctant of taking the Scudo for longer trips.

Please advice, thanks in advance.

Kind Regards,

Wouter