Outlook 14.4.8 on iMAC Retina and Yosemite 10.10.2 is not synchronizing with Exchange 2007 anymore (shows "downloading" but no new messages shown in inbox".

Accessing Mails with Webbrowser and from Windows Outlook is working correctly.

All Macs (also the ones with OS 10.6.8) do not anymore show any new messages in inbox.

Restarted Server, tried to disable autodiscover on macs and deleting/adding account and repairing database does not fix the problem. Also restarted server several times.

Any clue how to solve ?