I just saw this and I have the same issue, except that when I desactivate the plugin, I am left with another error message on my page:
Fatal error: Call to undefined function: imp_links() in /home/milk/public_html/st/wp-content/plugins/exec-php/exec-php.php(39) : eval()'d code on line 3
I have even tried to delete the plugin, but it hasn’t made no difference.