I had exactly the same issue with metzler karoos on a 100GS. There was a very slight airleak around the rim. I put some tyreweld in and it stopped leaking. When the tyre was being replaced there was no sign of any tyreweld left in the tyre. No subsequent tyre has had any problem on the rim. My best guess is that, that particular tyre wasn't a perfect fit for the rim.
I would not recommend using a tube in there unless it is an absolute emergency, they heat up too much and that can be problematic. Try tyreweld or some such product.
The electric monk always has faith.