Transferring a domain between registrars is a process that has nothing to do with Webflow. It occurs when you want to change domain registrars, or typically when a domain name ( or an entire company )has been purchased from a prior owner.
We help our clients through these processes often, and there are a few things we've learned to watch out for.
Before transferring a domain to a new registrar, make sure to disable DNSSEC.
If you do not, you'll run into problems later in the new registrar, when you may be unable to get DNS changes to take effect properly. Webflow's errors will be unclear, but it will be unable to complete the setup.
We've seen this happen most commonly in domains that were previously on Google Domains. Google Domains appears to add DNSSEC automatically to its domains and that feature "sticks" even when you transfer the domain to another registar. Many domains do not have DNSSEC as a feature, which means you then cannot remove the DNSSEC from the domain, and properly configure the DNS.
In the worst case situation, our go to approach has been to transfer the domain back to Google Domains, remove DNSSEC there, and then transfer it back to our new domain registrar.
https://community.cloudflare.com/t/request-to-remove-ds-record/477823
https://community.cloudflare.com/t/how-to-remove-old-ds-record/375925
https://community.cloudflare.com/t/dnssec-woes-request-to-remove-old-ds-record/362798/10
https://community.cloudflare.com/t/how-to-remove-ds-record-to-transfer-domain-to-cloudflare/206046/6
