The only fluid in the loop was bought from you:
http://shop.aquacomputer.de/product_info…roducts_id=2351
I emptied and refilled the loop shortly after it was assembled (to clean out possible leftovers from production).
That was in end march, so that was 5 months ago.
I recently refilled the loop, however, I have not emptied it since the initial cleaning run.
I guess a complete empty and refill is worth a try?