When a user attempts to SSO into the 365 they're presented with a blank white page and are unable to progress any further.
The browser cannot resolve ‘secure.aadcdn.microsoftonline-p.com’. This is a problem with the lightspeed filtering tool.
It had started categorising this Microsoft address as malware:
The URL needs to be accessible via lightspeed.
The address has been listed on their page before - https://support.office.com/en-us/article/office-365-urls-and-ip-address-ranges-8548a211-3fe7-47cb-abb1-355ea5aa88a2, however from a quick search it reveals that it has been blocked by various filtering systems before.
Change forwarders to 188.8.131.52.