Hello, I have the same problem, I also get the error "This is an encrypted OpenPGP message. In order to decrypt this mail, you need to install an OpenPGP add-on." But Enigmail is installed. A reboot did not help.
As a work-arround I've downloaded Enigmail from Debian 9, and installed it using "dpkg -i". I can encrypt messages without a problem now, but when I decrypt a message I get this alert: ------ Enigmail alert: GnuPG reported an error in the communication with gpg-agent (a component of GnuPG). This is a system setup or configuration error that prevents Enigmail from working properly and cannot be fixed automatically. We strongly recommend that you consult our support web site at https://enigmail.net/faq. ------ After clicking OK, I see the message decrypted. I have installed the package gnupg-agent, but I don't see pgp-agent in the output of "ps aux". With regards, Paul van der Vlis -- Paul van der Vlis Linux systeembeheer Groningen https://www.vandervlis.nl/