So i logged in again, and the tents had reappeared (and multiplied). We placed two tents, then another two after the first two disappeared. When I logged in, there were six tents.
But, all of them were empty, so even the items we placed into the new tents have gone.
And yes, I saved the tents each time.
I'm just gonna move server, friends have played on UK5 for weeks without any problems with their tents.