When you try to connect to the Central Management Server, the "Invalid Certificate" error occurs.
An "Invalid Certificate" error when trying to connect the Central Management Server usually means that the computer does not trust the certificate. Follow the Configuring HTTPS Certificate in Central Management Server article to add the GFI LanGuard Central Management Console CA as a trusted Certificate Authority or replace the default certificate with your trusted certificate.
Try to establish the connection to the Central Management Server again. If you still get the error, try adding the Central Management Server's hostname and the IP address to the Windows hosts file of the LanGuard server. If the issue persists, contact GFI LanGuard Support.