Modpack version
1.77

Issue
After a death, the client exits out from the server due to an "Internal Server Error". The server logs show an error in handling the packet. After some testing I found out that changing online-mode to false in server.properties causes the issue described.

Logs
I couldn't paste on pastebin since it exceeded the 512 KB limit, so here it is.
latest.log

I recommend you don't use that property :) Just enable whitelist and whitelist yourself only.