Scenario: On Windows, after entering the serial number for an Adobe CS product, you are instructed to sign in with your Adobe ID, but the installer claims it cannot connect.
Solution: Run certmgr.msc and then go to the following:
After clicking properties for the GlobalSign Root CA, choose "Enable all purposes for this certificate"
(If you don't see "GlobalSign Root CA", exit certmgr and visit THIS SITE, then close IE and reopen certmgr.msc)
Click OK and close out of the Certificate Manager, then have the Adobe installer retry the connection.