Client machine (windows 7) are not able to login in domain

Hi

I am facing one issue in my Domain:

I have windows 2012 DCs with with single foresta and 4 child domain and one 1 domain is existing tree (london.com).

Now in my existing Tree domain ((london.com). 

I am facing below issue:

Issue is that some of the users are not able to login and getting below error:
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event 5719
This computer was not able to set up a secure session with a domain controller in domain contoso due to the following: 
There are currently no logon servers available to service the logon request. 
This may lead to authentication problems. Make sure that this computer is connected to the network. If the problem persists, please contact your domain administrator.  

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Now all the client machines are in Existing Tree domain and are logining into another child domain that is contoso.parent.com

These all client machines are getting IP from DHCP and on DHCP server are below SCOP OPtion:

003 Router : Standard : IP address.
006 DNS servers: IP Address of Existing Tree domain (london.com)
015 DNS Domain Name : Standard : london.com

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Also when user machine connect from WiFi..... .. they are able to login in domain (contoso.com)

Also I have DNS Zones are forest wide replication. so all the ADDNS have same configuration in every child domain.

Now My concern is that this can be an issue if client machines are getting DNS suffix of london.com domain and ADDNS server as well and login into the contoso.com domain.

What I think here, in DHCP server of london.com, should be DNS suffix and DNS server of contoso.com

Please advice and help..