I think I've found the solution to this problem. I have no idea why ... but after uninstalling everything and screwing around with all of the folders, one by one, I discovered that FeralHeart randomly created 2 particle files in the FeralHeart\media\particles folder. They are named water_0.particle and water_1.particle. They were created just today by FeralHeart, no idea how or why. When I removed them from the particle folder, voila, FeralHeart started working again.
My suggestion to you is to check your particle folder for those files. If they are present, delete them. Your FH should work again once that is done.
On a side note ... what the hell happened to the OpenGL format FH used to boot up in? I noticed that that option was no longer available when I was looking at all the possible problems/reasons why FH was no longer working.