Minecraft version: 1.12.2
BuildCraft version: 7.99.15
Forge version: 14.23.2.2640
Link to crash report or log:

1. Assembly Table crash
2. Engine crash

Singleplayer or multiplayer: Multiplayer
Steps to reproduce:

1. Open Minecraft -> Log in to server -> Right-Click on Assembly Table
2. Open Minecraft -> Log in to server -> Right-Click on any BC engine (in my case Sterling)

Additional information: Tried disabling Optifine - no result.

On the off chance that his was somehow fixed in one of the releases since, can you try updating to 7.99.17?