I checked the page source code and it indeed has a Captcha. But it's an Invisible Captcha. For example, "Human users will be let through without seeing the "I'm not a robot" checkbox, while suspicious ones and bots still have to solve the challenges."
1) Either your IP is marked as bot automatically for some reason. Or, 2) if more people report the same problem here, they might have an error somewhere in their JavaScript code. Console log shows problems in /js/app.js file, presumably because of some crossorigin error, but I'm not sure these errors are related, because unique captcha IDs are generated on every page reload successfully. And 3) it might be that most people register, because they are not marked as bot, but, if it thinks you are a bot, it must show you the captcha, but the JS code, for example, has an error there...
Maybe try to register using a different IP? But I do not guarantee you won't be banned later... However, I sometimes do it like that on different trackers and everything continues to work fine. For example, I've registered on some reputable tracker with proxy IP many years ago and used it for several years until it closed.