This is caused by a corrupt email signature for the affected user, to resolve it you will need to:
Open User Profiles.
Locate and select the affected user's profile.
Click the Email/Printing tab.
Click Update.
Click the button next to the signature Name (the icon has a pen and paper on it).
Once the editing window has appeared click Accept.
Finally click Save.
If that does not work or you do not have access to User Profiles then please raise a new case online and reference the title of this article.