We're investigating a fix for the issue preventing people from signing into Skype 7.x on Windows Desktop.
If that is happening to you and you're willing to help us investigate, please let us know.
UPDATE: We think the outage which caused this issue is resolved, if you're still having troubling signing into Skype 7.x on Windows Desktop, please let us know here. If you're having sign in issue with Skype 8.x on Windows Desktop, please join us here: https://aka.ms/skype8windowsdesktop
The Skype Team