The game crashes when a fully grown weed plant is destroyed using flowing water.
We tested this in a singleplayer and in a multiplayer world and on both worlds we got the same crash report (Only with different Coordinates).
Destroying the fully grown weed Plant by Hand did not crash the game.
Destroying a not fully grown weed Plant with water did also not crash the game.
I have added 2 Crash Reports below (one from the Server and one when we crashed the game in a singleplayer world).

SinglePlayer: (in multiplayer it is the same)
-- Block being ticked -- Details: Block: minecraft:flowing_water[level=0] Block location: World: (-140,68,228)
(-140,68,228) is the location where the water source block is placed.

Other Information that might be relevant:
Minecraft Version: 1.10.2
Modpack: FTB Beyond 1.5.3
Map: Singleplayer: A new world
Agricraft Version: "2.0.0-0.8.0-a14"

Steps to Reproduce the Error:

1. Start FTB Beyond 1.5.3.
2. Create a new world.
3. Create farmland, place a crop stick on it and plant a weed seed.
4. Fully grow the weed plant using bonemeal (or wait).
5. Place a waterbucket near the plant so that it is getting destroyed by the flowing water.

Crash Report Multiplayer.txt

Crash Report SinglePlayer.txt

This may have already been addressed in-dev. I will have to check.

This has been fixed already in an upcoming alpha release.