Looks like me and XY have the same problem:
---- Minecraft Crash Report ----
// On the bright side, I bought you a teddy bear!
Time: 25/01/15 02:08
Description: Unexpected error
java.lang.NullPointerException: Unexpected error
at tconstruct.weaponry.client.entity.ProjectileBaseRenderer.doRender(ProjectileBaseRenderer.java:51)
at tconstruct.weaponry.client.entity.ArrowEntityRenderer.doRender(ArrowEntityRenderer.java:21)
at tconstruct.weaponry.client.entity.ArrowEntityRenderer.doRender(ArrowEntityRenderer.java:6)
at tconstruct.weaponry.client.entity.ProjectileBaseRenderer.func_76986_a(ProjectileBaseRenderer.java:20)
at tconstruct.weaponry.client.item.CrossbowRenderer.specialAnimation(CrossbowRenderer.java:64)
at tconstruct.client.FlexibleToolRenderer.renderItem(FlexibleToolRenderer.java:99)
at net.minecraftforge.client.ForgeHooksClient.renderEquippedItem(ForgeHooksClient.java:232)
at net.minecraft.client.renderer.ItemRenderer.renderItem(ItemRenderer.java:80)
at net.minecraft.client.renderer.ItemRenderer.func_78440_a(ItemRenderer.java:486)
at net.minecraft.client.renderer.EntityRenderer.func_78476_b(EntityRenderer.java:736)
at net.minecraft.client.renderer.EntityRenderer.func_78471_a(EntityRenderer.java:1361)
at net.minecraft.client.renderer.EntityRenderer.func_78480_b(EntityRenderer.java:1015)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:990)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:887)
at net.minecraft.client.main.Main.main(SourceFile:148)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
looks like crossbows might be off limits until they fix this.
EDIT: Having a lookup, you CAN NOT leave the server with a crossbow in your hand. Someone used NBT edit to change the item in the hotbar the guy was holding, but I'm not sure that this server has NBT.
If it doesn't, something like InvEdit or the modern equivalent should work if you edit the player inventory delete the crossbow.
Generally, highly modded TiC weapons seem to be a bit unstable... I had a steel scythe with 3 thaumium parts and all three mod upgrades, then 3 necrotic bone and 6 nether quartz that crashed me every other time I tried to hit a skeleton with it.
For me, my FTB crashed every time I hit a hostile mob with a scythe. BTW, could someone either edit my inventory to remove the crossbow, or (as a last resort) delete my player data? It would suck a lot to lose my Thaumcraft research+items, but it would suck more to never be able to play on the server again.