[Plugin: WP Single Post Navigation] Some function breaks the plugin

Hello,
I’ve got a problem with a function in my theme and your plugin and I cannot determine wich function is faulty.
The problem: next and previous nav link generated by the plugin are always pointing to the same posts.

I use a child theme of DotB theme. Your plugin works nice with DotB so I know it’s my functions.php that contains the guilty function but I connot find wich one – too much errors when trying to desactivate and sometimes, the plugin works without logic.

So, here is my functions.php http://pastebin.com/V0BTsNT8
If you got an idea of the guilty function, I will try to find a solution.
And thanks for sharing your work.

http://wordpress.org/extend/plugins/wp-single-post-navigation/