this is a typical error which comes out when SnapDrive is not configfured correctly, as per the Administration Guide and therefore is not fulfilling all requirements.
I suggest you read the requirements in our documentation.
Additionally, if you navigate to the SnapDrive installation folder on your windows hard disk, then open the SnapDrvdbg.log just after the error occured, scroll all the way down, then read backwards: you will see that there even a link to a KB which explains how to tackle this kind of problems.
Most likely this is a name resolution issue between SnapDrive server and the source and/or destination storage system ,but could also be that you are not using correct credentials, as the error message suggests.
In fact RPC transport Protocol Setting is more demanding than just using http, you may want to setup SnapDrive to use http, instead. RPC requires cifs service to be running on storage system and that you setup SnapDrive service to run with a user which can be authenticated on storage system (locally, if it's a workgroup or through Kerberos, if both Storage and Windows are in the same domain). In other words, it's easier to configure SnapDrive with http, as you just need to setup one (admin) user on the storage system and configure SnapDrive with that user and password.
the scheduled backups were running fine before. and the installation of snapmanager was done about 2 years ago in our current working environment, it is very recently that the backups have started to fail.
initially we thought that its a UID and password authintication issue. So we tried to fix that but still the issue persists.
can there be other reasons as well that can cause this??
if you haven't recenlty changed the password for the SnapDrive (domain?) account and nothing changed in DNS or local windows hosts file, then it's most likely a problem with the cifs status on the storage system: the storage system needs cifs service to be running and working fine within the workgroup or the domain. If not, it won't accept any RPC calls.
As a matter of fact, I do see some CIFS related errors on your storage system:
Thu Jul 2 01:00:57 BST [sunmisysf01:nbt.nbss.socketError:error]: NBT: Cannot connect to server 10.80.49.117 over NBSS socket for port 139. Session setup error. Error 0x82: Called name not present.
Thu Jul 2 01:00:57 BST [sunmisysf01:cifs.server.infoMsg:info]: CIFS: Warning for server \\MISYS-DC008: Could not make TCP connection.
Thu Jul 2 01:00:57 BST [sunmisysf01:cifs.server.errorMsg:error]: CIFS: Error for server \\MISYS-DC008: Error while negotiating protocol with server No Trusted Logon Servers Available - STATUS_NO_LOGON_SERVERS.
You may want to take a close look at that.
try cifs testdc, cifs domaininfo or (if you can afford a cifs downtime) cifs terminate and cifs restart.
You have the option httpd.admin.enable on, which means you can configure SnapDrive to talk http to the Storage system. why not give a try?