Polycom FW version 5.5.3 and Exchange hybrid.
From the logs we see that Polycom tries to perform autodiscover, but finally fails and leave EWS not deployed for online users. I will continue debuging that, but I found a workaround:
login with sip and UPN as email@example.com but leaving domain field empty.
Then in settings-applications-exchange server URL you can hardcode:
and leave autodiscover disabled.
Update: with CURL DEBUG we can see that problem was in SSL certificates of Digicert not being trusted due to limits we left from other tests in Network - TLS - Application profile 6 (select certificates from platform or imported, but we had only one certificte selected). I also imported Digicert root, as for unknown reasons outlook is based on that while rest of MS is Baltimore.