Hit by the same issue after updating to the latest Microsoft Office 2011 security updates.
I discovered that if I launch Outlook 2011 (14.5.0 _ 150423), go to Preferences, select Accounts, and remove all of my IMAP accounts (in my case 3), and restart Outlook, the interface window re-appears.
I added the IMAP accounts back (gmail, hotmail, isp) via Preferences and discovered that upon adding a third IMAP account, the interface window problem occurs. I removed the third IMAP account that was created and had to use the Activity Monitor to "Force Quit" the Outlook "SyncServicesAgent" in order for the Outlook interface window to re-appear.
I tried different IMAP account order and it seems that the problem occurs upon the creation of the third IMAP account. If someone has POP and/or Exchange accounts, it would be worthwhile testing on a problem system to determine if it's specific to IMAP.
SIDE NOTE
I'm using the latest version of iTunes on a fully patched Yosemite on a MacBook Pro and on an iMac. I typically sync Calendar/Contacts/Notes from Outlook 2011 via iTunes. I've noticed that this is no longer working as well ... it possible that the "SyncServicesAgent" may be the culprit.
Regards,
HGagne