There are a few reasons why you may get a "SSL expired" error message, which are:
1. You're trying to activate the free SSL option when signed up through a hosting partner. The free SSL option can not currently be activated through a hosting partner and requires that the domain be signed up directly for Cloudflare pointing to our nameservers.
2. You removed and then added the domain to your account. This will cancel out the earlier request to issue the certificate through our vendor, which can take 24 hours to activate, so that is going to lead to an expired certificate error message. We will automatically attempt to reissue the certs in an expired state, but please take note that this process may also take the standard 24 hours or so for to activate through our vendors.
3. The domain is not pointing to our nameservers. Provisioning of certificates on free Universal SSL requires the domain on our nameservers as a check from our vendor, so the certificate will expire if you move the domain away from Cloudflare.