I know its something that you would never [realistically] do, but i randomly did it, so here it is. I placed a double slab from blood arsenal that i cheated from JEI, and placed it.

What i find weird is that VanillaFix blames your mod for it and not another mod that I have like kneeslabs... but here is the report:

client
server

If its not your issue, I will make sure to report it at kneeslab's repo next