Whatever key is bound to the Enable/Disable Item Search option disables the bound key in chat. I discovered this just this afternoon when I couldn't type 'Z' after installing the mod.

I did some testing and can confirm that it only happens to this specific binding (the other two bindings don't affect chat even if you pick letters or numbers) and this happens to ANY letter bound to the Item Search option. I changed it to A and was suddenly unable to type A in chat while I could type Z just fine.

Minecraft 1.9
Forge 12.16.0.1854
MoreOverlays 0.6