OK, I have a very weird issue here. 2 sites, 3 DC's in each site (1 physical, 2 virtual).
Site A absolutely fine, Site B member servers and client PCs all report this in Event Viewer;
Level: Error
Source: NETLOGON
Event ID: 5719
Description: This computer was not able to set up a secure session with a domain controller in domain <DOMAIN> 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.
ADDITIONAL INFO
If this computer is a domain controller for the specified domain, it sets up the secure session to the primary domain controller emulator in the specified domain. Otherwise, this computer sets up the secure session to any domain controller in the specified
domain.
Level: Error
Source: GroupPolicy
Event ID: 1129
Description: The processing of Group Policy failed because of lack of network connectivity to a domain controller. This may be a transient condition. A success message would be generated once the machine gets connected to the domain controller and Group Policy
has succesfully processed. If you do not see a success message for several hours, then contact your administrator.
Now I would normally be very worried, except logging on is fine, using the computers is fine. It's a if there's a problem just before offering up the Ctrl + Alt + Del prompt then everything is fine.
Except because of this the machines in this site do not get software deployed to them through Group Policy.
I have tried:
Removing from and rejoining the domain
Static IP address
Changing switches
Re-installing Windows on a PC
Moving all FSMO roles to a DC in the 'problem' site
Do a meta-data cleanup using ntdsutil (couldn't do anything as no missing DCs were detected)
Member servers are also affected.