My NextCloudPi on Pi 3B+ has nicely upgraded itself to 1.15.2, the Raspbian base to Buster and Nextcloud to version 16.0.3; comfortably without any real effort from me. Big thanks to the developers!
Now when I access the system with SSH it tells me I have new mail. What is this mail and where can I find it?
This has nothing to do with Nextcloud and is a Raspberry Pi OS specific message. You should be able to read the system mail using the mail command line program.