I did not have this particular problem, however there was one strange thing I noticed in this mission.
The German soldiers manning light mortars (those ordinary, infantry ones) in the upper left corner, behind the first Tiger from the left, were invisible. They appeared only after receiving damage, or more precise, after I blindly fired a HE round at their position. They are part of the enemy defense, so maybe they are the problem.
Naturally, I assumed they had some secret Nazi personal cloaking devices, so we stripped them naked and sent their clothes back to divisional HQ.
PS btw, its quite possible to form a functional defense line, if you defend in front of the base during first two phases.