The specified domain either does not exist or could not be connected

When I try to logon to my Root Domain (root.com) on Child Domain Controller machine (tango.child.root.com) using Root Domain Administrator account (administrator@root.com) it gives me an error that "The specified domain either does not exist or could not be connected"

But sometime it allows me to logon. I am not able to understand this issue at all.

Replication between my Parent domain controller and child domain controller is working fine.

I have checked my network also and there is no issue in my network.

Child Domain Controller Name: tango.child.root.com

Child Domain:child.root.com

Root Domain (Forest): root.com