When placing a sign (any type) on an empty block (any type), everything works as expected, the sign is placed and you can type.

When placing a sign (any type) while "looking" at tall grass, the sign open event is not triggered - the sign is removed from the inventory and placed (even when in creative mode) - and the following error happens in the chat.

https://hastebin.com/efamofoxax.cs

GIF of the problem: 

Full server log when the GIF was made: https://hastebin.com/kodumodaxa.sql

The problem does not happen on single player using the same client version (Pre5)
The problem does not happen on a multiplayer Pre5 vanilla server