Nextcloud 12 internal server error trying to access admin page

Nextcloud version : Maybe 12.0.3. Don’t recall the exact version as I can’t access the admin page.
Operating system and version: Unknown. Probably a very recent Debian version
Apache or nginx version : Unknown.
PHP version: 5.6 (Select-able to 7.1 on my shared hosting)
The issue you are facing:

Logged in as administrative user, I want to access the administrative page to update my NC installation. But it ends up with an internal server error.

Is this the first time you’ve seen this error? No

Steps to replicate it:

  1. Regular installation from install script on shared hosting. (Back when 12.0.3) branch was the latest.
  2. Create an administrative user, install applications
  3. Create regular users for current usage.
  4. Log back in with administrative user when an update is released.
  5. Some users will experience the Internal server error.

The output of your Nextcloud log in Admin > Logging:

Snipped showing only the part relevant to the request ID displayed on the error page.

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

_This file isn't in "path/to/nextcloud._

The output of your Apache/nginx/system log in /var/log/____:

File is not accessible on this shared hosting.