NextCloudPI: 403 Forbidden on web interface, no SSH enabled

Nextcloud version 18.0.7
Running on NextCloudPI (not sure which version)

The issue you are facing:
The system has been running fine for a while but now suddenly, when I try to log in via the web interface I get the 403 Forbidden error. Unfortunately SSH seems to be disabled (I don’t think it’s on by default and I’m pretty sure I didn’t enable it)

What’s the best way to fix the install?