I have been playing a Modpack for a few weeks with the AquaAcrobatics mod in it alongside the Essentials mod so i can play with a friend and just recently today Essentials had an update and after the update Minecraft would crash after pressing play. I looked into the logs and found AquaAcrobatics was the problem and was conflicting with Essentials.
launcher_log.txt

Essential hacks in to the game too much and is known to cause random issues with mods. I'll accept a contribution that fixes this but probably won't be fixing it myself.