I traced it, but I can only see that the query isn’t returning any result.
MapPress uses the WordPress query functions, so I can’t tell you if it’s a problem in the plugin or something else in the blog.
Here’s some suggestions to narrow it down:
1) Deactivate ALL of your other plugins for a minute, and switch to a standard theme like 2012. If that works, then it’s a conflict with another plugin (WPML, perhaps?)
2) Rather than using the widget, try putting PHP to generate the map directly in your template’s footer:
<?php echo do_shortcode('[mashup query="p=23"]'); ?>
Also, add a line to just print out the query result – that way you’ll know if it’s an issue with the map or with the query:
$wpq = new WP_Query(‘p=23’);
echo “Query: “;