Encoding troubles on RSS page

1. Go to Forum, log in
2. Create a post with cyrillic letters
3. Press RSS button (the "cup sign")

As a result, the list of posts for the last 24 hours should be shown, cyrillic letters are shown in a wrong way (see attachment)

Expected result: the list of posts for the last 24 hours should be shown correct, cyrillic as well as latin

This behavior doesn't happen when an unregistered user presses the RSS button