WordPress MU

I'm having this issue.
When i try to upload a file with the flash upload after the stage "Crunching..." shows me a login screen (wp-login) from my blog inside the admin area (i don't get redirected, i'm still in the admin area but it shows me the square to login)..
Image: http://img263.imageshack.us/img263/4238/44669495.jpg
Only happens with flash uploader, the Browser uploader works fine. Wildcard enable and working, but this is happening in the main blog also.
No plugins activated.
I'm running wpmu in private server, vhost on. also the temporary file is created in php tmp directory, i can't find wich the error is...

Any ideas?

thanks!

I've found the solution. Cookies problem. Cleared the browser cache and problem solved.

Also please check to see what version of Adobe Flash that you are using. Later dot versions of 9 and early dot versions of 10 had issues with the uploader. Not sure if that problem would apply in this case but I wanted to mention it.