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!

make sure you have Make fast RPC calls? set to on (when adding the server) and port 10000 to 10010 are open