It is not the service account, but the logon account whose credentials you need to verify. You can check logon accounts, by going to Network-> Logon accounts on Backup Exec console. Make sure they are same on both the servers. Also, verify the logon account used in "resource credentials" tab in the backup job properties. This logon account must be granted privileges on the target server being backed up. Change password on the logon account, to make sure you are using correct password. Also, ensure that system logon account is present in the logon accounts.
Please mark it a solution, if this is useful.
Thanks