I encountered the same problem. I solved this by
1) moving all the ppas from the folder /etc/apt/sources.list.d to the file /etc/apt/sources.list. I did this with the command:
pr -F *files > newfilepr -F *files > newfile
which combines all the sources.list files into one file. Some adaptation is needed.
2) removing *all* the files in /etc/apt/trustedgpg.d
then, I could add new keys to the keychain by issuing the “sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys **KEY_HERE**” command (look it up if you don't know it). Before that, it didn’t import the key. Hope this is helpfull to someone, it was a completely random move on my part, had no idea if it would work or not. It did :D