A quick fix is to edit db.inc (for example, if you use MySQL, it would be sql/mysql/db.inc) and change:

function q_bitor($fieldLeft, $fieldRight) {
	return $fieldLeft .' | '. $fieldRight;
}

to

function q_bitor($fieldLeft, $fieldRight) {
	return '('. $fieldLeft .' | '. $fieldRight .')';
}

The problem doesn't occur on all PHP stacks, only apply it if you get the same problem.