I thought I'd share my experiences here. I upgraded from Drupal 5 to 6 and I was getting a blank XML feed and the following error message:

warning: Parameter 1 to theme_slideshowpro_xml_image() expected to be a reference, value given in /var/www/vhosts/mysite.com.au/httpdocs/includes/theme.inc on line 668.

For me, the solution was to change "&$node" to "$node" line 93 of slideshowpro.theme.inc in the following line:

function theme_slideshowpro_xml_image($node, $size = 'preview', $title_caption = 0) {

To be honest I am not sure what this is all about, but it works...