It was in the Word docmanagement forum, but given the passage of time the
reply was

"This is probably caused here by Adobe trying to write to the same space
that
your custom toolbars occupy or from interference with another add-in. eg the
Works add in is notoriously problematic if you have that -
http://www.gmayor.com/multiple_mail_merge_entries.htm

Try making your custom toolbars float, allow Adobe to write its toolbars to
the toolbar areas then put yours back in a different place, leaving
Acrobat's where they automatically go. It is probably not a good plan to add
personal icons to the standard toolbars. Use custom toolbars instead.

See also the last section of
http://www.gmayor.com/lose_that_adobe_acrobat_toolbar.htm"