1) Enter a Debug-File under Extras/Options in the tab 'Extras'. 2) Try to get the categories. 3) Send me the Debug-File (don't worry, Login/Password are masked) to [support at blogdesk dot org] 4) You can now delete the entry for the Debug-File in the options.
WordPress internally calls a file named class-IXR.php - but on your server this file is called class-ixr.php. If your server is case-sensitive it won't find the file (because ixr <> IXR). Solution: rename the file from ../wp-includes/class-IXR.php to ../wp-includes/class-ixr.php.