NB> It seems as though he's trying to actually prefix his directory name, ie:
 NB> fido_, So Mystic seems to be telling him that
 NB> ../mystic/files/fido_ doesn't exist. I've always understood the "Prefix

It removes the prefix part and checks to make sure the base directory is
accessible.  If it cannot access the base directory then it shows that error.

--- Mystic BBS v1.12 A46 2020/05/28 (Windows/64)