There was no crash-log saved anywhere. Neither on my pc, nor on the server. The only message I could find was in the ftb launcher console:

[11:28:53] [Client thread/WARN] [FML]: ****************************************
[11:28:53] [Client thread/WARN] [FML]: * Null fluid supplied to fluidstack. Did you try and create a stack for an unregistered fluid?
[11:28:53] [Client thread/WARN] [FML]: * at net.minecraftforge.fluids.FluidStack.<init>(FluidStack.java:35)
[11:28:53] [Client thread/WARN] [FML]: * at net.minecraftforge.fluids.FluidStack.<init>(FluidStack.java:67)
[11:28:53] [Client thread/WARN] [FML]: * at codechicken.enderstorage.storage.liquid.TankSynchroniser$PlayerItemTankCache.getLiquid(TankSynchroniser.java:225)
[11:28:53] [Client thread/WARN] [FML]: * at codechicken.enderstorage.storage.liquid.TankSynchroniser.getClientLiquid(TankSynchroniser.java:264)
[11:28:53] [Client thread/WARN] [FML]: * at codechicken.enderstorage.common.ItemEnderStorageRenderer.renderItem(ItemEnderStorageRenderer.java:48)
[11:28:53] [Client thread/WARN] [FML]: * at net.minecraftforge.client.ForgeHooksClient.renderInventoryItem(ForgeHooksClient.java:183)...
[11:28:53] [Client thread/WARN] [FML]: ****************************************
[11:28:53] [Server thread/INFO]: KayKusanagi lost connection: TextComponent{text='A fatal error has occured, this connection is terminated', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null}}
[11:28:53] [Server thread/INFO]: KayKusanagi left the game