406 Not Acceptable

Hi together

I always get the message “406 Not Acceptable: Due to a security rule based on the transmitted data the request could not be accepted. Security configuration is modifiable in the control panel.” from my server, if I click on the button “Add File” under “Downloads”.
Does anybody know what I have to adjust that this plugin works correctly?

I am using WordPress 3.5.2.

Thanks in advance for your help!

Kind regards