This error rarely occurs when you have updated CorelDraw X6 to 6.1 and trying to log in via the membership and subscriptions.
Error: Unknown Error
A reason for this error is the Windows Auto Update is turned off and the root certificate authority is not up to date.
Update the root certificate of your Windows.
Then, please do this:
Open Internet Explorer,
Go to the advanced tab.
Scroll down to "check for server certificate revocation" and uncheck it.
Apply the changes and close Internet Explorer.
(If you do not have Internet Explorer, go to Control Panel>Internet Options)
Restart your system,
launch CorelDraw X6,
then try logging in.
Please try it out using CorelDraw X6 as it will log in other software included in the Suite like PhotoPaint X6.
You should now be able to log in without errors.