Since you are using symantec antivirus, that might be your answer. It looks like others are having the problem as well

This is an issue with Symantec Endpoint Protection 11.

Symantec has created two patch files, one for 32bit operating systems and one for 64bit operating systems, to resolve this issue.

Download Outlook_SEP86_MR1_1378.zip for 32-bit operating systems
Download Outlook_SEP64_MR1_1378.zip for 64-bit operating systems


To resolve this issue without using the patch there are two steps:

* If you upgraded from Symantec Endpoint Protection 11 to Symantec Endpoint Protection 11 MR1 then you will need to disable the Outlook plug-in both in Symantec Endpoint Protection and in Outlook.


To disable the plug-in from the Symantec Endpoint Protection Manager (SEPM):
1. Open up the AV policy in SEPM.
2. Click on Microsoft Outlook Auto-Protect
3. Unselect Enable Microsoft Outlook Auto-Protect

To disable the plug-in from within the Symantec Endpoint Protection (SEP) client interface:
1. Open up the SEP client UI.
2. On the AV bar, click Options and then click Change Settings...
3. Click on Outlook Auto-Protect
4. Unselect Enable Microsoft Outlook Auto-Protect

To disable the plug-in from Outlook:
1. Open Outlook
2. Click Tools -> Options
3. Select the Other tab
4. Click the Advanced Options button
5. Click the Add-in Manager button
6. Unselect the Symantec AntiVirus Outlook Protection


* If you installed Symantec Endpoint Protection 11 MR1 directly without upgrading from Symantec Endpoint Protection 11 and the above steps do not work, you will need to go to "Add or Remove Programs" to remove the AntiVirus Outlook Plug-in.


Steps to uninstall the Outlook Plug-in:
1. Open up the Control Panel
2. Open Add or Remove Programs
3. Select Symantec Endpoint Protection
4. Click Change
5. When the installer comes up click Next
6. Select the Modify option and click Next
7. Open the AntiVirus Email Protection
8. Click on the Outlook Scanner and select the This feature will not be available option
9. Click Next and then Finish