Wordpress failed to import XML files

Hello WPMUdev Team.

I have the following problem: When i try to import an XML file i get several 'import failed' errors after i select the post authors, as you can see here:


After i took the capture, i tried deactivating all plugins and using twentytwelve and then uploading the XML file but i still get the same errors, and nothing gets uploaded.

My php max execution time was set to 60 and i changed it to 500 but i think thats not the problem because im still getting the same results.

Hope you can help me, Thanks!