Ah, I have a lead for you while I add the error message to the UI!
On the camera integration page:
- Open the browser inspector (right-click → inspect element), « Network » tab
- Click « Test » on the problematic camera
- Look for the request in red, it should be « api/v1/service/rtsp-camera/camera/test »
- In the response, there is the error returned by ffmpeg (the tool we use for cameras)
Well, this is really just a temporary solution. On my side, I will add the error in the UI, it will be simpler 
Note: this does not work on Gladys Plus, only on your local instance.