502 Bad Gateway error in Nginx Proxy Manager with 'migrations_lock.frm' permission denied in logs

Hello,

Checklist
Checklist

Have you pulled and found the error with jc21/nginx-proxy-manager:latest docker image?
Yes, 'jc21/nginx-proxy-manager:2.12.1'
Are you sure you're not using someone else's docker image?
Yes
Have you searched for similar issues (both open and closed)?
Yes

Describe the bug
I am experiencing a 502 Bad Gateway error in Nginx Proxy Manager when trying to access my upstream server. The upstream server is accessible directly, but when accessed through Nginx Proxy Manager, it returns a 502 error. I checked the logs in Docker (nginx-proxy-manager-app-1 container) and found the following error:

Nginx Proxy Manager Version
2.12.1

To Reproduce Steps to reproduce the behavior:

1- Configure a new proxy host in Nginx Proxy Manager with the IP and port of the upstream server.
2- Attempt to access the configured domain.
3- Observe the 502 Bad Gateway error in the browser.

Expected behavior
The upstream server should be accessible through Nginx Proxy Manager without errors.

Screenshots

Operating System
Running on Ubuntu Server 22.04 in a virtual machine.

Additional context
Docker version: 26.1.4
Browser version: 129.0.6668.58
This issue also occurred in previous versions of Nginx Proxy Manager (2.11.x) and was not resolved by updating to 2.12.1.

Best regards
Matias

No source file or test is named. Start by reproducing the issue with the jc21/nginx-proxy-manager:2.12.1 Docker image and inspect the container logs for the migrations_lock.frm permission error; done requires identifying whether the failure is in Nginx Proxy Manager, MySQL permissions, or the upstream configuration.