try changing the code:
action='plugins/<?php echo $_SERVER['PHP_SELF']; ?><?php echo (isset($_SERVER['QUERY_STRING']) && $_SERVER['QUERY_STRING'] != '')?'?'.$_SERVER['QUERY_STRING']:''; ?>'
to:
action='plugins/<?php echo $_SERVER['PHP_SELF']; ?><?php echo (isset($_SERVER['QUERY_STRING']) && $_SERVER['QUERY_STRING'] != '')?'?'.str_replace("&","&",$_SERVER['QUERY_STRING']):''; ?>'