Please click "Tools"->"Preferences" menu item in Extension Manager, check "Enable detailed logging" option. Restart Extension Manager. Try to install the extension. After the error message pops up, quit Extension Manager. Send the log files (including ExManCoreLibrary.log) under "/Users/<YourUserName>/Library/Application Support/Adobe/Extension Manager CC/Log" folder to firstname.lastname@example.org.
Note that the "/Users/<YourUserName>/Library" folder is hidden by default. Please hold down "option" key and select "Go"->"Library" menu item to go to "/Users/<YourUserName>/Library" folder.
Log files sent. thanks