I asked what these error codes means in WHMCS / Subreg / Gransy domain registar module.

"Empty required fields [505:2002]" - this one is claimed to be related to the additional parameters (eurid registry requires parameter "lang")

"You are not allowed for this domain! [501:1004] Probably not yet registered" - This error appears thanks to the first error message mentioned above. The reason for it to appear is because the domain was not yet registered.