wrong character encoding in privatemsg message subject and body

Hallo,

There is something wrong with character encoding for japanese characters sent by privatemsg.

If I send a private message with japanese characters in the subject or the body of the message the preview still displays them right but after sending the display of the sent message or the title shows only question marks ('?????' instead of 'にほんつう').

Could there be a wrong setting in my data base? I noticed that the collation of my drupal data base in general is: utf8_general_ci.
But the collation of subject, message and hostname in the privatemsg and privatemsg_archive tables is: latin1_german2_ci.
I think that might be the error. But is this a bug in the privatemsg.module or installation? And could I simply change it to utf8_general_ci without damaging the messages?

What can I do?
Thanks in advance.