I’d guess that you have switched (or were switched) to newer customer accounts, but the theme only supports legacy customer accounts or needs to be switched to use them?
Once you have switched to the new account, there is no going back. That’s a good thing - it’s more secure. In addition, there are many requirements that Shopify has to meet in many countries. It is better to focus primarily on one system than to have several systems running in parallel.
Yeah this happens when new customer accounts are turned on but something’s misconfigured. Go to Settings > Customer accounts and switch it back to the classic accounts option instead of the new one. That’ll fix it right away and people can register again.
To fix the registration error you’re seeing, check the following:
Open Settings and go to Customer Accounts. Make sure “New customer accounts” is checked (not turned off).
Check your theme. If you’re still getting the error while using Dawn, your theme files may still have old account URLs like (/account/register). Instead, these need to go to the new login system.
Get in touch with Shopify Support directly. Since you confirmed that even a new Dawn theme has the problem, it seems like something on the account configuration side that only Shopify can fix from the backend.
It’s better to get this right than to try to go back to the old way of logging in (email → one-time code). I hope this helps!