NOTE: If you are using 'WebRoot' as an Anti-virus product, disable (remove is preferred) and try again. For reasons unknown (and only addressable by them), WebRoot will prevent MP from working properly consistently.
NOTE:: You cannot connect to a Windows XP machine via KVM mode. The errors will be that that it is a firewall issue or that KVM mode is not enabled.
NOTE: All testing should be done via Ethernet (not wireless) and directly connected to a single router. Also, if used, any VPN connection should be disconnected.
Ensure you have the latest and same version of MP software installed on each machine; one as a Primary another as a Secondary. You can check your version on the 'Help' or 'About' tab in MP. You can get the current version on your account page:
https://www.stardock.com/support/productkeyretrieval
Next (and important), try rebooting every MP machine, as well as the connected router, and testing the connection again:
Seamless:
KVM:
If that fails, and as a test, shut of Windows Firewall (as well as any other AV \ Firewalls you may use):
Try your connection again.
If it works, turn the Windows firewall back on, download the following file (on all MP PCs), and run the included batch file:
https://cdn.stardock.us/support/uploads/MPPortFix.zip
If that works, it was a firewall port issue.
If that fails, you need to check that the firewall profile you are using is set for Domain, Home or Private, NOT Public.
Changing Windows network type should you need to:
Win7:
http://windows.microsoft.com/en-us/windows/choosing-network-location#1TC=windows-7
Win8:
https://www.youtube.com/watch?v=KRBXOuCWk18
Win10:
https://tinkertry.com/how-to-change-windows-10-network-type-from-public-to-private
If that still fails, removed the Secondary connection completely:
Seamless:
KVM:
And re-adding the connection by the Secondary's IP instead of the host name:
Example:
