I'd like to add that if I leave the value of trash-directory to nil, then I believe the default system trash directory is created somewhere in my /home.
Because then, based on my earlier email, CASE A (deleting directory in my emacs user dir works fine, but CASE B (deleting directory in my /tmp) gives that error.
So far, at least on my machine, it looks like if
- delete-by-moving-to-trash is t, AND
- trash-directory is in /tmp and to-be-deleted-dir in is HOME (or vice-versa)
this error occurs.