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Some entity is inside a block which causes the server to crash, I suppose that would be your friend. You can try removing all the blocks around where he last was
Yes I Suspected That Because It Says That An Entity Hit A Block And It Only Crashes When He Joins
A Few Days Ago The Server Started To Always Crash, And Today When I Joined It Worked! 
But Later When My Friend Joined It Crashed 
Heres The Log:
https://mclo.gs/kGbstEw
Edit: Also It Says Vanilla But Im Running A Modpack The Modpack Is Trollcraft
