I'm having the same issue. Today I tried to update your plug in to 4.0.4 and it crashed my site as well as the wordpress work space for my site. Any time I try to move to a different screen in http://www.momentumsc.com/wp/wp-admi, I get this error message:
Fatal error: Call to a member function get_option() on a non-object in /home/momentum/public_html/wp/wp-content/plugins/yet-another-related-posts-plugin/yarpp.php on line 54
I had just updated some posts and checked to see that they are working and was prompted to update your plugin. That's when this trouble started.
To answer your questions above to Colir see my answers below:
1. Have you used YARPP before on this WordPress site? yes
2. Did you install this by copying the files into wp-content, or using the builtin plugin installer? plugin
3. If you updated, did you update together with other plugins, or did you just update this one? with two other plugins (Page Links To and ShareThis)
How can we fix this? I need to have my site up and running again.