Can you post the entire log? It looks like your registering an override for Minecraft clock. If you could post your code as a GitHub repo that would be nice.
Overriding getAttackDamage (or similarly named) will give you more control over the damage, there’s nothing wrong with your way though.
Don’t use static initialisers! Instantiate your items in the registry event. If you need static references to your items, use @ObjectHolder
Where’s your packet? I think you might be calling code from one thread (the packet handler thread) that modified stuff in another thread (the Minecraft thread) causing a concurrent modification exception