Diacritics in subject tag

we have a problem with diacritics (ěščřžýáíé) in Subject Tag (sample ticket id: [Žluťoučký #9999]). When you create a new ticket via e-mail and then you reply to the “autoreply e-mail”, the reply is not added as a comment to the existing ticket, but a new ticket is created instead. When you replace the letters with diacritics with plain ascii characters, everything works fine and subject of the reply is properly parsed and comment is added to the existing ticket. We found this problem in RT version 4.2, but after upgrade to 4.4 the problem persists. There’s no problem with displaying these characters, so I assume it’s not a collation problem in database.

Does anybody have an idea what did I miss in my configuration?

Thank you in advance.