I am uploading doucment using object model. However my 'UploadFile()' method throws exception "The request failed with HTTP status 403: Forbidden.". This happens if my current user is not a member of one custom group (ABCGroup). And 'ABCGroup' has all permissions. Even if I give 'Full Control' permissions to this user it doesn't work. So I am really wondering.
As you are trying to upload files through code, did you try to use the RunWithElevatedPrivileges method ? Below is a link which will give you more information on the method.
You can do something like this: