Placing a chain on a fence on a 1.16.5 server ( java 8 mohist #468 ) cause a crash with this message ( all players are disconnected too ) :
https://imgur.com/a/7tUVsiR

1.16.5 is no longer supported, sorry

Exact log when the crash happen : https://pastebin.com/c6Kutvxf
Forge version used : 36.1.4 and 36.1.0