The doors have an inner and outer layer of fiberglass, with varying types and amounts of core between the layers. Water can get in where there are breaks in either "skin," such as for the window, hinge attachment fasteners, or door handle.
It's possible that these fixtures weren't well-bedded to begin with, or that the bedding has simply aged past the point of doing its job.
I come from a boating background, and on fiberglass boats the decks are often constructed similarly. There it provides a structural function (i.e. makes the decks stiff and light), so it's really important to keep the core dry and healthy. Once it gets wet and the skins delaminate, you no longer have the "I-beam" structural function.
door is not structural in the same sense, although the stressed skin could help to keep the door curved. (That said, I'm not sure if this was part of the engineering thought when they made them, as I've read varying reports of what's actually in the doors, ranging from full core, to strips, to nothing.)
To keep water out of the inner door, the most basic step would be to "bed" the fixtures with something like butyl tape or caulk. On boats, we oftentimes ream out the core for a short distance around the item and then fill the gap (or larger hole in the case of a fastener) with thickened epoxy. Once that cures you have a barrier (in the case of a fasterner you redrill the smaller fastener hole through the new epoxy annulus). Then you reinstall the item and re-bed it.
I realize there are various reasons for sagging doors, but if the body is "straight" and the hinges aren't pulling it out of alignment --- meaning the the only problem is the bottom of the door "relaxing" its curve --- then "re-coring" the door would most likely fix the sagging. It's the same as the boat decks: The two skins spaced apart by the core, but held fast to it, create a strong, shape-holding structure.
Not saying you need to do all this on your Boler
door; I really just started to explain where the water got in, since you were wondering, then I got a bit carried away