However when I try to register a new account called zajac in the CLI:
register_account zajac TEST652Ynn6QAP518Po2GM7t8Ws5gJGTyFYBsfyvNr9KjzH2pgJZaT TEST652Ynn6QAP518Po2GM7t8Ws5gJGTyFYBsfyvNr9KjzH2pgJZaT neura-sx neura-sx 0 true
Looks like you need to import the active key of neura-sx.
//Update:
Yes I checked it's the reason. Although I don't know why the API still need active_key when you already have owner_key. Will check when have time.
Thanks [member=18687]abit[/member] , you are right, I forgot about the active private key.
After importing it everything works fine.
I'll prepare another round of FAQs and publish them together with answers I got so far from xeroc, abit and others.