Does the crash appears on client ? I've got this bug, the problem was I used the Minecraft.getMinecraft().thePlayer, and it returns a EntityClientPlayerMP. On server, this class doesn't exist, that's why it makes crashes. I think it's not related to the zip format. This is deprecated, but I think it works. But the jar format is better, and Forge Gradle uses this format, why do you convert it in zip format ?