I still have not resolved this issue yet. The plugin appears to be correctly installed in Administration-Plugins. Another oddity: the plugin appeared in the "Add new object" dropdown menu as "-rep_robj_xpho_obj_xpho-". This issue seems to have settled, because now it appears correctly as "Fotogalerie". But the "PHP Fatal error: Cannot redeclare class ActiveRecord" remains and the page turns blank, when I try to edit an existing or add a new gallery.
Help is still greatly appreciated!