Any idea why I can't register to this instance from the webui?

Might be because the server load of new users is too heavy right now. I'm still learning the fediverse, but I do know that it somewhat doesn't matter what instance you register with as long as the other instance is federated with the one you registered in you'll be able to access and see all posts.

Sorry for the long sentence, but hopefully that helps. Here is a link to how the fediverse works on a wiki: https://joinfediverse.wiki/Main_Page