For me, Lync conversation history was not getting displayed in outlook (a Form region cannot be opened).
Issue got solved after removing the entries
HKEY_CURRENT_USER\Software\Microsoft\Office\Outlook\FormRegions and by restarting Outlook.
Follow the instruction at link :
Outlook has certain built in forms that can become corrupted at times.
In the most recent case, this issue happened when a user was trying to
view their Lync messages in their Conversation History folder within
Outlook, and was greeted with the "Outlook Form Region Cannot be
Opened" error. Of course, they can double click the email to view the
messages, but will not be able to view it in the Email preview.
Follow these simple steps to resolve this issue with the Outlook
Forms. This can be done under the user's profile, and does not need
administrative privileges. Proceed with caution when working with the
Registry Editor on your PC. Follow the instructions carefully so you
do not accidentally remove the wrong key.
Close Outlook (This has been tested on Outlook 2013, but should be valid on earlier Outlook versions as well.)
Open up the Registry Editor - For Windows 7, Go to Start Menu > Type regedit in the search box. For Windows 8 PCs, go to your Metro Start
screen and simply just start typing registry editor and the search
will pull it up for you.
Navigate to the following registry key:
Just in case, right click FormRegions and select EXPORT, then save this .reg file to your desktop in the event you have to revert back.
Delete the entire FormRegions folder from the above registry location
Now, Reopen Outlook and try viewing the messages in the Outlook preview to make sure it opens correctly now.