Homemade Liqueur Help

I'm making my first batch of "cherry bounce" - a cherry-infused brandy - using an old recipe from William Woys Weaver's "Sauerkraut Yankees" with instructions from a friend who has had success with the recipe before. One month ago, I combined about 2 quarts of pitted sweet cherries with 1 cup of sugar (maybe more sugar, I don't remember) in a clear, plastic jar, and covered it with brandy. Per the recipe, I also added the cherry pits, tying them in cheesecloth so they weren't loose in the liqueur.

The jar sits on the counter in my kitchen. I tied cheesecloth over the top, and everyday I stir the mix. Yesterday morning, everything looked fine. Yesterday evening, the top developed a scum and the mix was bubbling!

My kitchen can get warm, and the jar was sitting in the light - I'm not sure if that's what caused this. I talked to my friend and she hadn't experienced anything like this with her cherry bounce. The scum reminds me of the kind crocked sauerkraut develops. With kraut, I just skim it off, but the bubbling of the liqueur concerns me.

What went wrong, and is there anything I can do to save the bounce?