Installation fails with failed to run self-test with auto-generated config or database access denied

I just tried to install notify_push on my server.
Server info:
Ubuntu 21.04 running in KVM on an AMD Epyc server
Plesk 18.0.36
Nginx Reverse Proxy
Apache Webserver
PHP 8.0.7
Nextcloud 21.0.3
MariaDB 10.5
Redis set up using a socket instead of a port (working fine)

Installed the app from the store. notify_push:setup gave the following error:

Allowing self-signed certificates in the push config. 🗴 failed to run self-test with auto-generated config. test output: [2021-07-01 21:07:49.054589 +02:00] INFO [notify_push] src/main.rs:49: Running with certificate validation disabled
and the following in nextcloud log:
Error: fsockopen(): Unable to connect to localhost:7867 (Connection refused) although the port is not in use according to lsof.`

So I tried the manual path. I created a systemd service file according to the readme (changed ExecStart and username). I added

location /push/ {
	proxy_pass http://localhost:7867/;
	proxy_http_version 1.1;
	proxy_set_header Upgrade $http_upgrade;
	proxy_set_header Connection "Upgrade";
	proxy_set_header Host $host;
	proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}

to my nginx "additional instructions" portion in Plesk, but got an access denied error:

Error:
notify_push[102401]:    0: Failed to connect to Nextcloud database
notify_push[102401]:    1: error returned from database: 1045 (28000): Access denied for user 'nextcloud'@'localhost' (using passwo>
notify_push[102401]: Location:
notify_push[102401]:    /tmp/krankerl.P1VaKOfPDklj/notify_push/src/storage_mapping.rs:57

my trusted proxies are also configured correctly (127.0.0.1, ::1 and my servers external IP).

I do not know what else to do.

Start with the notify_push:setup entry point and the installation instructions in the README, then inspect src/storage_mapping.rs:57 alongside the reported systemd and Nginx configuration. Reproduce the self-test and database connection errors in the stated Nextcloud, MariaDB, and proxy setup; done means the installation path completes and notify_push connects to the database.