Invalid RPC login problem

Deleting server uh10.esw.cz .. Deleting mail aliases .. .. done

Turning off virus filtering .. .. done

Turning off spam and virus filtering .. .. done

Deleting MySQL database uh10_esw_cz .. .. done

Deleting virtual website .. .. done

Deleting DNS zone .. .. done

Deleting slave DNS zone on ns.easysoftware.cz .. .. done

Removing from email domains list .. .. done

Deleting home directory .. .. done

Deleting server details for uh10.esw.cz .. .. done

Updating Webmin user .. .. done

.. done

Applying web server configuration .. .. done

Re-starting DNS server .. .. done

Re-starting slave DNS servers .. .. some slave servers failed

ns.easysoftware.cz : Invalid RPC login to ns.easysoftware.cz

Re-loading Webmin .. .. done

What can cause that virtualmin once logins right and another time it fails?

Thanks a lot!