Problem solved !
after i have written the post above, i remembered this situation in artus topsite:
after walking (and fighting ;-) ) through the canyon the at-st hangar door opened and an at-st came out. The first time (the time the problem appeared) i ran back and activated the first stationary gun. After waiting for a while, no at-st came around the corner ... a security look round corner showed ... no at-st around anymor, it just dissapeared !
So i started the level again and just ran besides the apaering at-st into the hangar and took the door to the lift towards the oin-cannons. After deactivating the shields of the cannons i entered one of the three guns and destroyed the at-st, that was at this time still inside the canyon.
Then i destroyed the ion-cannons and went back into the hangar ... voilą the door has been opened by some troopers.
Best regards !