Using melted hard candy, you are at the mercy of the ingredients of the hard candy manufacturer.
I like to make my own sugar glass.
Here is my recipe for sugar glass: 1 C Water 1.75 C Sugar .5
C Corn Syrup -
the clearer the better .25 t Cream of Tartar
I always supplement it with cream of tartar and corn syrup.
The cream of tartar stabilizes the mixture and the corn syrup helps the glass stay clear.
Sucrose (table sugar) is a disaccharide (two sugars) of glucose and fructose.
Most cracking occurs when when there is a structural defect due to crystallization.
Applying acid (cream of tartar) causes the sucrose to break down into its component sugars and stabilize. Also remember that heat and humidity are the enemy of sugar glass.
Do the best you can to control the environment of the sugar glass.
cooking.stackexchange.com/users/21831/BigHandsome
I don't consider ExpertChef's answer directly applicable, but it shows an interesting experiment possibility.
It could be that glueing a tough, flexible film to your sugar windows will absorb some physical tension or jolting effects, and/or
prevent drying out which leads to cracking.
(The second shouldn't be a problem with traditional hard candy, but we don't know what your recipe is made of).
You could make such a film from high concentration gums in water (don't bother flavoring, at that concentration aroma isn't really noticeable, and the lack of sugar probably won't register against the sugar window).
There was some molecular gastronomy recipe for packing butter pieces in a transparent "hull", if you can find it (I couldn't :( )
whichever hydrocolloid they used will probably work well for you.
I don't know if it is better to brush the wet hydrocolloid on the window and let it set, or to make the film first and then glue it, this needs experimenting.
As an alternative to a gum film you could try glueing rice paper or oblaten to the inside of the windows.
It is probably the easier way out.
But it should be a more or less strong bond, just sticking it with jam or similar won't keep the sugar part from experiencing physical stress.
These are all untried ideas, if somebody has applied them, I would like to hear results.
cooking.stackexchange.com/users/4638/rumtscho
Skip the melted candy windows!
It's not worth the stress of wondering about the amount of humidity in the air
and when the windows will start melting and running in riverlets down the side of your beautiful gingerbread house and then disappear completely, leaving open holes where your windows were.
Use sheets of gelatine instead.
They look awesome, stay good FOREVER and actually look like old fashioned leaded windows with the little diagonal diamond shapes.
I make gingerbread houses every year for my confectionery and find the finest, tiniest strings of battery operated lights which I fix to a fondant Christmas tree standing in the "garden" before threading it through under the wall to the inside of the house.
So beautiful.
cooking.stackexchange.com/users/53036/Sherene
You should try making them with isomalt instead of table sugar.
It is often used for decorative sugar work, for many reasons, but structural stability is one of them.
cooking.stackexchange.com/users/1393/Michael Natkin
When I make glass windows for my ginger bread houses I just crush hard candies, put the sides of my house on a cookie sheet with a thin smear of oil where the cut out windows are.
Then I put the crushed candy in the cut out windows of the sides and front of my house.
I put it in the hot oven, watch it closely, and wait for it to melt.
Then I have candy windows.
When it's cool they come right up without cracking because I put a small amount of oil under them.
Then I put my ginger bread house together.
I also put a light under my houses
so it looks like someone is "home".
cooking.stackexchange.com/users/22451/Anita
I have melted hard candy in the windows of my gingerbread house, two days later the candy is cracked.  Tried it again, using more candy, but it cracked again.  The gingerbread wall with the windows was baked and sat for two days before the hard candy was melted.
How do I keep my melted candy windows in my gingerbread house from cracking?
cooking.stackexchange.com
<candy><gingerbread>
cooking.stackexchange.com/questions/38040
cooking.stackexchange.com/users/21015/Sheryl
This can happen because of structural problems from crystallisation, or because of environmental factors. Use isomalt instead of table sugar or try glueing rice paper, oblaten or sheets of gelatin to the inside of the windows. Alternatively, you can just leave the windows unglazed. Glass candy windows can crack and to prevent this it is necessary to control the environment. It may be better to use an alternative such as gelatin. Or perhaps not make the glass windows.
Glass candy windows crack due to crytallization so it is important to control the sugar glass environment. Alternatives to candy glass include gelatin and using isomalt instead of sugar. Do not make candy windows.