What does the /install page on your server show? The videos are rendered on your server so it sounds like ffmpeg or imagemagick isn’t working (or maybe it’s tied to the same issue as the URL).
The URL sounds like the test ID in the json file is getting overwritten on the relay server. I have a couple of systems that are successfully running as relay clients but it’s possible that something with the key format (mine don’t use the A.* keys) is messing things up or it was a bug that was fixed and I’m running newer agent code.