External Access to Traccar-Website results in Error: Unexpected token ... is not valid JSON

Hi @all,
hope you can help me with a hint. I use Traccar in a docker-container on a synology NAS with an MariaDB/SQL.
When I open the Traccar-Website in my local-network on http://IP-NAS/8082 there is no problem. When I try from outside I get the message:

Unexpected token '<', "<!DOCTYPE "... is not valid JSON

The configuration to access from outside is:

• Cloudflare Domain with published Application
• Authentification via MFA before reply to Traccar website
• SSL from Cloudflare to OPNSense, internal http-routing
• With other published applications (Paperless, Photoprism) it work without any problem

A few weeks ago it worked without a problem with traccar, too.

Perhaps somebody have a hint for me to search for the error-code? I don't know where to start the debugging.

Best regards
David

Have you checked the details about the error? What's the request and response?

Without Cloudflare-Authentification before accessing traccar it works. Other Applications on the same host have no problem with the pre-integrated MFA. So I don't know where to search the problem.
What do you mean with your question, what could I check?

I'm talking about HTTP requests.

That mean? Can you give me a hint what I should check?

Not sure what kind of hint you need. The API request failed. I'm asking you to check all details from that request, not just the error message. You can use the browser developer tools for that.

And I said I have no idea what I can do and with your short answers you don't help me. Thats ok, I hadn't asked you for help. I am not a web-developer.

Perhaps there is someone in this forum who has the same use-case with cloudflare, that would help. I don't believe that all self-hosted Traccar-instances are without MFA.

Your case is very unique and even if there are people who do something similar, it's a very small percentage and it's very unlikely that they will read this thread. If you don't need my help that's fine, but I guarantee you that you will need to get your hands dirty if you really want to get to the bottom of this problem.

But I am not hopeless :-)

And I have no problem to "make my hands dirty" but as I said - I don't have the necessary know-how to analyze it.

Open your browser developer tools. There should be a network tab. Then load the website and check if you get any errors. Those are usually marked as red, but depends on the browser. Inspect the error details.