Here is a log file for N900 attempting to sync to a Kerio 6.7.3 Patch 1 server for the first time and receiving the "Error communicating with Exchange server".

SSL is working at least as confirmed by Modest successfully connecting to server via IMAP.
I've changed username to MyUserName.
There was also a shortened, three letter, version of my user name. That also was replaced with MyUserName.
Any reference to my server has been replaced with example.com.

Grep'd on term error and found: 5, 6, (3, 901)

Saw that DeviceType=SmartPhone on one of the lines and this matches to what I see in the Kerio admin interface. Would be nice if it read N900 or Maemo 5, as the other devices state their proper name such as iPhone, Nokia5800XpressMusic, iPod, etc.