These rumors have now materialized into the official communication below,
The reason for the decision to postpone the pre-launch, is that the LEGO Group’s quality department has found that a pneumatic cylinder included in the set does not live up to the LEGO Group quality demands, as there is a risk of the cylinder leaking air. Since the LEGO Group wants to ensure a high quality in all products as well as a great play experience, it has been decided to replace the pneumatic cylinder in all new sets.
The launch in retail-stores will remain August 1st.
There is only a very minimal probability that consumers, who have already purchased an 8110 LEGO Technic Unimog U400, will experience difficulties with the pneumatic system."
Fortunately this time, the unlikely potential problem has been detected before the model to reach the mass market and a significant amount of costumers had the opportunity to purchase the new model. Hence, eventual complains have been prevented just in time.
Also the 1st of August is already too close.