I've added a line after "authenticate();" in showframe.php to fix the problem with AJAX and encoding, as I am using windows-1251:
header ("Content-Type: text/html; charset=windows-1251");
This way the problem with cyrillic and photopost/ajax has been resolved.
Maybe it will be very nice if the header encoding in showframe.php is changed by the maintainers in the next releases. So that I may not need to modify the script again. 10x