Just a little back round: I just recently changed out my cabin and air filters. When reinstalling my cabin filter, I had a hard time getting the back tab to seat properly. So I suspect that it may not be in all the way because the rear of the filter is not

 

You'd be best off taking more time and trying to get that sucker seated correctly. Get a friend with smaller hands if necessary.

To answer your question about the dampness inside the car, I don't think the cabin filter would have contributed to this, but water has a way of doing a lot of weird things.

This diagram might help you understand why I don't think the incorrectly cabin filter would have attributed to you problem:
http://www.hondaautomotiveparts.com/auto/j...4s2a0_b1710.gif

 

If the filter isn't firmly in place, its not in place. Its a minor pita, but you can get it in there - just take a peak and see how it needs to seat.

 

I live in VA, so I change cabin filters very often. Usually every year regardless of miles.