NIC.es regulations sets restrictions when registrant is a person, not company. Nevertheless, when you set Company as registrant you can't set personal contact as administrative contact.
The alert message is shown:
Attention: if registrant is a natural person, registrant and admin contacts must be the same.
Such behavior was classified as the software issue PBAS-30274 and was fixed in OBAS 4.5.6 release.
To resolve the issue, please upgrade to 4.5.6 as described in How to upgrade OBAS system to the latest available version.