WordPress: page not found bug

Hi, since I upgraded from 2.0.4 to 2.0.5 I keep getting errors when editing or posting a post, page or options. It tells me that the document or page can’t be found and it shows me the URL. Changes have been made if I refresh it and it takes me to the page it normally would. I’ve tried uploading en overwriting everything on my server but that didn’t help. Any ideas? I’m using the latest K2 (stable) theme. Thank you.