HTTP 500 when requesting panels in admin

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I'm coming back to C5 after a few years and is having some installation/configuration issues.

Platform: Nginx, PHP-FPM 7.2, Chrome.

Installation went pretty smooth and I got pretty urls and so on working.
The page loads and I can access the admin. However whenever I want to access the side panels or editing a block, jQuery hits upon a HTTP 500 error, when the content retrieved being:
{"error":{"message":"Access Denied"},"errors":["Access Denied"]}
.

No messages in the php log, nginx error log and so on.
It's a clean install. Nothing has changed and file permissions should be OK as well.

Any ideas?