Hey Matias! Welcome to Discuss, and thanks for your patience.
I hope you’ve been able to solve this already, but if not, I’d recommend resetting webhooks, checkout keys, and deploy keys associated with the project. That will help us make sure you’re working from a clean slate.
Try the steps in the Support Center article below. Once that’s done, refollow the project on CircleCI, which should allow builds to start again.
This fixed the problem for me. One detail I noticed was that, before I changed anything, the repo that CircleCI was having trouble checking out didn’t actually have a Deploy key listed. Once I set the webhook up again, it did have a Deploy key, and the checkout worked as expected.