ln has several syntaxes. man ln 1 ln:

   SYNOPSIS
          ln [OPTION]... [-T] TARGET LINK_NAME   (1st form)
          ln [OPTION]... TARGET                  (2nd form)
          ln [OPTION]... TARGET... DIRECTORY     (3rd form)
          ln [OPTION]... -t DIRECTORY TARGET...  (4th form)

so without -T or -t what is target and what is link name is ambiguous and
ln tries to guess. Now imagine:

    ln -sf /path/to/new/hook    $H

and let us consider that $H is already a symlink, pointing to some place
which _exists_, but current user do not have access to. Then ln will
complain:

    ln: accessing `$H': Permission denied

and abort.

Fix it by specifying ln form we use explicitly with -T.