My guess is the Falling Leaves mod, not sure what's wrong but it's a first mention in the crash report. Could be the files got corrupted or aren't loading correctly since you mention it was working at one time. Could also be a certain type of image that wasn't loaded until you entered a new area where that image was used or collected an item that uses it that's now in your inventory.
Try placing a new copy of the mod in your mods folder or see if there are any updates for it.
Could also see if the author of the Falling Leaves mod has any idea.