in order to solve this issue  you should check the following:

1. Make sure you set 777 permissions under the following folders:

/admin/tmp
/admin/logs/
/admin/xml/
/images/
/media/
/music/
/xml/

2. Make sure your server is allowing Content-Type: multipart/form-data; to run PHP with POST method. If no then you should contant your host to allow it.

3. Make sure your server is allowing Content-type: application/x-amf; to run PHP with POST method. If no then you should contant your host to allow it.