If this error occurs at registration time, please ask your provider for assistance. It means they are rejecting the registration for unknown reasons.
1) SIP ALG or firewall on the router
SIP ALG (Also called SIP PASS THROUGH or SIP HELPER) was invented to get around NAT problems by rewriting some packets.
Unfortunately, SIP is rather complicated and most SIP ALG implementations are horribly broken. (Netgear, Dlink, Zyxel).
If you experience missing audio, failing registrations, failed calls, failing video calls, failed transfer, please verify that your router does NOT use SIP ALG and disable SIP ALG it if it does.
If you are unable to change the router configuration, try using TLS encryption (if the SIP server supports it) to encrypt the traffic and stop the router from tampering with the packets.
2) The provider does not allow frequent re-registrations.
Please contact your provider and ask what value to configure for the re-registration time settings in Zoiper.