Fabrizio,

The Jobo tanks/lids have small plastic fingers that all need to be contained then tank is closed. It is pretty easy for one to be bent and on the outside. This will cause the tank to leak. I always check for this after loading roll film. Depending on the light in your darkroom, you may need a small flashlight to see in there. Prewash is generally not recommended for E-6 and I would try to solve my leak before I started processing.

Chuck