Hashnode uses Vercel to manage custom domains. This means if you add a custom domain on Hashnode, and later try to use the same domain on Vercel, you will see an error. For example, if you have added blog.foo.bar on Hashnode, you may not be able to use foo.bar on Vercel.
Please follow the steps below to fix this:
1. Remove the custom domain blog.foo.bar from Hashnode.
2. Add foo.bar on Vercel.
3. Come back to Hashnode, and add blog.foo.bar.
4. You will get a confirmation email from Vercel to delegate the control of blog.foo.bar to Hashnode. Just click on it, and confirm.
5. Hashnode will now be able to issue SSL for your custom domain, and serve it as usual.