Hi, I’m having the same problem without a solution: Gmail SMTP Email Server config
back in December I installed the newest 15 version from the scratch (Ubuntu, Nginx, MariaDB, PHP) in a test server just to see how it looks and I didn’t have that issue reproducing exactly the same exactly the same configuration using Gmail and the same Letsencrypt certificate for my domain and it worked perfectly, there must be some thing from Ubuntu side that I cannot still figure out.
In my production environment I installed from the scratch Ubuntu 16.04 with Nextcloud 13 and the upgraded to 18.04 and NC 14 then afterwards to 15 and the issue is still there…