Calling hook OnItemCraft resulted in a conflict between the following plugins: CraftMultiplier - False (Boolean), InstantCraft (True (Boolean))

No idea though if this plugin needs fix or InstantCraft.

I don't know why he would modify the original way the plugin works and return true.
I can't do anything about this, I have to be compatible with two other plugins and I can't change the return value to true

Did you ever find a work-around for this? I keep getting the same console message.