My friend keeps getting kicked from my server. reason: Threw an exception at the server.

[Lamorak]

Every time he speaks to a trader he is immediately removed the server. can someone please help? the only reason it gives is: "Threw an exception at the server"

[Samus_Aran]

Is it modded or unmodded?

 

If it's unmodded, try submitting an official bug report on github.

If it's modded, try removing mods one at a time and trying again, until it stops crashing, then report the issue to that specific mod.