It is happening as ServiceNow SSL certificate is expired so it is navigating you to http:// rather than https://. Many other people are affected by this and hope it will be fixed soon by ServiceNow.
Work Around: - Open developer.servicenow.com in incognito mode and sign in and launch PDI. It will display error, but it will also display Advanced option towards footer. Click on it and on next screen click on "Procced <PDI URL> unsafe" link and you will be able to access your PDI.
Regards,
Nikhil Bajaj
