iCloud email aliases unavailable in Mail
After upgrading to 10.11, my iCloud email aliases are no longer available in Apple Mail (under the pull down when you choose the "From" address). I have tried adding them in the Preferences>Accounts>Edit Aliases, but it hangs in the dialog box and it never saves. However, the aliases are still listed when I login to iCloud.com, so they are still present and active.
MacBook Pro (Retina, 15-inch, Mid 2014), OS X El Capitan (10.11)