This solved my problem: http://www.phpbb.com/phpBB/viewtopic.php?t=210139. But it only works for new messages. To fix the messages that already had been posted before the fix was applied need te be edited and saved (you don't need to change anything in the message) to get quotes work for that message.
Yes! Thanks. It solves my apostrof problem and quotes are working now indeed (but the messages which containes quotes that are posted before this fix is applied need to be edited and saved again (without changing anything) to get that quotes work correctly..