Avatar Manager problerm

Hello to all.
I have istall the Avatar Manager but when i insert the image and select the upload botton give me the follow error:
—————————
Page Not Found

HTTP 500

Most likely causes:
• The website is under maintenance.
• The Web site contains a programming error.

You can try:
– Refresh the page.
– Back to the previous page.

Further information Further information
—————————
What has happened? How can I fix the problem?
Thanks for the support

http://wordpress.org/extend/plugins/avatar-manager/