Midway through function `org-agenda-align-tags', there's a call to
`add-text-properties' that attempts to take an existing text property
and add symbols `org-tag' and `face' to it. Since around org-mode
version 5.17 or so, I've noticed problems with the lists formed there --
particularly on XEmacs 21.4.21 (Cygwin build).

Sometimes the existing text property already contains the symbol
`org-tag'. Sometimes the existing text property is already a list;
sometimes it's just an atom. I'm guessing the intent of the forms in
`org-agenda-align-tags' is ensure that `org-tag' is part of the text
properties; if it's already there, we should leave it there.

Toward that end, I've found the following change makes things work as
expected.