New webhook subscription error

[KN]

Hello,

I am trying to subscribe to webhook events, but I have the following issues.
I get an error: "GET to callback URL does not return 200", so I did a test, as mentioned in Troubleshooting Tips, by sending a GET request to my callback URL. The test worked fine and the response is under 2 seconds. 
I would appreciate some feedback on this matter. I have also attached 2 images showing the process. In the first image, there is a POST request to the subscription endpoint with a status of 400, while the second is a test to my callback URL with a status of 200.
Am I missing something here? Thank you for your help.

[veloek]

It's a bit difficult when we can't see more of the URLs in your screenshots, but I have a couple of (stupid) questions:

1. Have you configured content type application/json in your response?
2. Does it work if you omit the trailing / in the callback_url?

[KN]

Hello, thank you for your reply. Calback URL is less than 100 characters long and have multiple paths and one query param.
1. Yes
2. I tried with and without
Same error.
Thank you.