Air flow is the key. Removing the plywood is the first thing to do this will allow air to carry the moisture up to the upper vent and out of the building. As it is now the plywood is trapping moisture against the roof causing frost.
If you want to eliminate the problem for good and the areas are accessible spray foam insulation is exceptional. It would eliminate the need for vents at all. I live in an old home 1800's and cut the draftiness almost totally out by using spray foam against the roof deck and around the band board in the basement.
I realize this is a much more expensive fix but I believe it would be permanent and ultimately save you money on your heating and cooling along with making the home much more comfortable.