NextcloudPi is an awesome way to get an instance of Nextcloud running on you Raspberry Pi and in a short time. It's as simple as downloading an .img file, burning it to an SD card, and boot up your RPi (you still need to enable SSH for a headless install).
However, there are some things I've come to find repeatedly troublesome while setting up the server. The single most annoyance for me is the built-in Let's Encrypt set up. Though it's an awesome feature, it fails for me every time if I forget about turning off the 'force https' feature.
I do most of my work via the command line, however, you can do the same via NextcloudPi's setting web interface on 4443 (192.168.1.xxx:4443).
If I try running the 'letsencrypt' module from the config page in the terminal, I'll get a fail message if I attempt to get my certificate without any other changes.
What I've found is that you need to first scroll down to 'nc-httpsonly' and shut this off.
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After you've shut this off go back and try to run the Let's Encrypt module again, it should work (remember to have your NextcloudPi RPi on port 80 and 443. After this, you can point those ports wherever you need to and use a reverse proxy if needed).
I hope this helps, at the very least this can serve as a reminder to me.