Error on login to WP from certain browsers

Warning: Invalid argument supplied for foreach() in /home/devgrou1/public_html/opa/opanew.ohpsych.org/wp-content/plugins/lock-posts/lock-posts.php on line 173

Warning: Cannot modify header information - headers already sent by (output started at /home/devgrou1/public_html/opa/opanew.ohpsych.org/wp-content/plugins/lock-posts/lock-posts.php:173) in /home/devgrou1/public_html/opa/opanew.ohpsych.org/wp-includes/pluggable.php on line 897

Only on certain browsers and seems to be random. The last error was from safari. But I did get this from firefox this morning
http://opa2011.ohpsych.org/wp-admin/