Strange… but the message can also mean that there’s something wrong about the Chest itself (its folder, its index file, …). Is it possible to move some other file (eicar, for example) to Chest?
He said that after reboot everything was fine and avast! found only 1 file which was later properly moved to Chest.
I doubt this helps to you at all, but i just couldn’t leave the post unanswered…