There is currently a limitation of how many certificates you can register per week: a quota of 5 per domain per week.

The same limitation applied for renewal, which would have forced to maintain a schedule.

This is not the case anymore: if a certificate has already been generated for a specific  FQDN, you can renew it regardless of your quota use. Thank you for Roland Bracewell Shoemaker for this change, which solves this issue.