To ensure you and your users do not lose access to Lync Online, Microsoft has published a blog post to ensure your network is allowing a new Certificate Revocation List (CRL). This is required because of the acquisition of VeriSign by Symantec. Failure to allow this CRL can result in the inability for your clients to connect to Lync Online.

Here is the information from the Blog Post:

Location Protocols Outgoing TLS and HTTPS

Add this new rule to your firewall or proxy server to avoid Lync sign-in issues and ensure a secure connection to Office 365.   

IMPORTANT: You must add this allowed CRL endpoint to any device that is filtering web traffic: firewalls, load balancers, proxy servers, web security software/appliances, and so on.