I have Debian Stretch installed on USB booting Raspberry Pi and working well. I installed via the steps in https://pimylifeup.com/raspberry-pi-nextcloud-server/ and was able to connect with the desktop client from two of my Macs. I then upgraded to version 15 not knowing that I may have needed to proceed through v14 first.
At the end of the upgrade, I was met with the window that I needed to upgrade to 15.0.8 but the upgrade window gave an error and could not complete. Now, when I go to raspberrypi.local/nextcloud or 10.0.1.4/nextcould (my internal static IP), I get HTTP ERROR 500. 10.0.1.4 is unable to handle this request. raspberrypi.local shows the index of / including nextcloud/ which links to a black browser screen.
I come from a Mac world with some intermediate experience in Terminal. My Linux experience comes from the Raspberry Pi where I am a novice.
I can initialize my USB and begin with a new Debian install if that is better than undoing the mess I may have created.
Mark