The problem occurs when a user tries to add an attachment to a Web Email Protection message doing the following:
Create a new message or reply to an existing one.
Click on the paper clip icon or on Add attachment.
Click on the Browse button to find a file and double click on it.
Click on the Attach button.
This type of problem is acknowledged by Microsoft in the Known issues with this security update section of MS13-055:
After you install the security update for Internet Explorer 9 or for Internet Explorer 10 that is described in Microsoft security bulletin MS13-055, you may experience the following issues when you access some websites:
A browser window automatically closes and logs you out of a web session.
Drop down menus are not correctly populated.
No error is displayed for the user and the Symantec Encryption Management Server Web Email Protection log shows only the following when the user with email address email@example.com is replying to a message and trying to add an attachment:
This issue is caused by the KB2846071 hotfix which is described in Microsoft Security Bulletin MS13-055 published on 2013-07-09. Many users of Internet Explorer 10 will have it applied automatically via Windows Update. Users with Internet Explorer 9 who have installed the update are also affected.
Use the following steps to determine if KB2846071 hotfix update is installed.
Click Start > Control Panel.
Under Program and Features, click View installed updates. The Installed Updates are displayed.