The command "svn cp . svn+ssh://..." tries to copy a directory that has
just been removed and fails:

  $ svn -m "[svn-buildpackage] Tagging libhtml-tree-perl 4.1-1" \
    cp .  svn+ssh://svn.debian.org/svn/pkg-perl/tags/libhtml-tree-perl/4.1-1
  svn: Directory 'debian/patches' is missing
  svn: Directory 'debian/patches' is missing

The directory debian/patches has been removed before (using svn rm) and
has been commited to the repository without complains:

  $ debcommit
  [...]
  Deleting       libhtml-tree-perl/debian/patches
  [...]
  Transmitting file data ............
  Committed revision 64356.

I did try to tag the upload right after this via svn-buildpackage
--svn-tag-only which runs the command mentioned above.  Trying to run
the command by hand fails as well.

Regards,
Ansgar

PS: I had similar problems after merges as well.  After "svn merge",
"svn commit" believed some files were outdated (it fails for one, moving
the file aside caused the commit to fail at another file).  "svn update"
did also mark the file as "conflicting" (not introducing any changes),
but "svn resolve --accept working $file" did not help.  It removed the
conflict marker, but the commit still failed...