This kind of problem can be caused when there is something wrong with the central authority certificates on your system. These certificates are used to establish a chain of trust from someone Ubuntu or Mozilla trusts to a particular site when https: is used. The chain of trust starts with certificates installed as part of your Ubuntu system.
The problem might ordinarily manifest itself when you use a web browser to make a secure connection to a web site. In this case
add-apt-repository is using the https: protocol to securily add a repository and its authentication keys. The current Ubuntu ppa repositories require a chain of trust from MakeDaddy.com to Canonical. If you use Firefox to browse to the same repositories using https: you will probably see an error rather than a closed padlock for the site, and clicking that will provide error messages indicating that the chain of trust cannot be determined.
Enter the following to reinstall the central authority certificates:
sudo apt-get install --reinstall ca-certificates
Another helpful command for this: