409 Conflict Error on Login via /login Endpoint in Self-Hosted Docker Setup

[Kodalinq]

Hi team 👋

I'm encountering a 409 Conflict error when trying to log in via the /login endpoint. Here's what I’ve observed:
The request is a standard POST to /login.
The server responds with 409 Conflict and the message:
"The request could not be processed due to a conflict."
The response appears even before any form validation is triggered.
Request is sent via browser from https://tracker.my-domain.com/login.

Headers and environment details:

I’m using the application via Docker proxying it through Nginx proxy manager, and also using Cloudflare proxy as a second proxy.
I have also tried disabling proxying and logging directly into the software with no results
I haven't made any changes to the authentication logic manually.

Remote address (as seen in DevTools): [IP_addr]:10809
strict-origin-when-cross-origin seems to be enforced.
Possible involvement of CSRF or session mismatch.

What I've tried:

• Cleared browser cache/cookies
• Disabled browser extensions (proxy/VPN related)
• Tried in incognito mode and another browser

Could you help me understand what may be causing the 409?
Happy to provide any configs or routes if needed.

Thanks in advance 🙏