Deployment of add-ons makes trouble

Hi,
the deployment of add-ons from the Moodle plugin database does not work. In Moodle 2.5rc1+ (20130513) I get the following error message.
Ralf

Oops! It did it again

Moodle deployment utility had a trouble with your request. See the docs page and the debugging information for more details.

exception 'download_file_exception' with message 'Unable to download the package' in mdeploy.php:761
Stack trace:
#0 mdeploy.php(1520): worker->execute()
#1