What will cause excessive condensation in the AHU and duct system of an AC system?

Presently system is saturated with water, which has leaked into duct insulation, and stands frozen due to cold weather conditions.

Lennox AHU AC system in attic installation. AC unit only. Separate electric heat.

2 Local installation contractors have suggested heated air from living space is entering duct system and condensing. However the amount of water present makes this answer unlikely in my opinion.
Also, homeowner indicates that filters are wet and moldy at changing.
Homeowner indicates AC system is extremely efficient, i.e. cools house very fast.

Could system be 'overcharged' to cause freezing of coil thus producing excessive condensation in ducts...

By reading your question it sound like you are running your AC unit with low outdoor temps. Are you using it as a humidor or in a wine cellar? Overcharged will not freeze the coil, lack of air flow, undercharged or trying to cool at low temps will. Need more info.

You can't believe how much humidity can migrate into cold ductwork and condense? Lets me count the ways, Heat rises and moves to cold, humidity migrates to a drier location, les saturated. The ductwork may get down to as close as 15 degrees of the outside temperature, or more, in the winter i.e.. 5 degrees outside, 20 degrees in attic. Cooking, showers and humidifiers of course, add to this problem. I have seen flex ductwork get so heavy it fell through the drywall. It is a real problem. The last case I stated, the solution was. To remove and replace all the duckwork. While we where there we installed a safety drain pan and brought the installion up to code. Provided the homeowner with magnetic sheets for the supplies and a ductboard winter block, for the cold air return. At least 5 years later never a complain, we got a very nice letter about here heating cost being significantly better from the previous 2 years she lived there.

By reading your question it sound like you are running your AC unit with low outdoor temps. Are you using it as a humidor or in a wine cellar? Overcharged will not freeze the coil, lack of air flow, undercharged or trying to cool at low temps will. Need more info.

Thank You
Running at low outside temp was another suggested cause, however homeowner is absent in cold weather and thus unit is not run.
Homeowner states they actually run unit sparsely. Perhaps 5 -6 times last summer as house is located at relatively cool 45th parallel.
'Improper' charge sounds like the most likely cause.

Thank You
KC