Troubleshooting Outlook Form Corruption
Weird things happening or unexpected errors with custom Outlook forms you've developed in Outlook 2007 or 2010? If so, it could be Outlook form corruption.
Try following the instructions in the link below:
The relevant part of this is:
Clearing the Forms Cache
The first step in troubleshooting is to use the command in Outlook to clear the cache using the Manage Forms dialog (Tools | Options | Other | Advanced Options | Custom Forms, then click Manage Forms).
If that does not resolve the problem, shut down Outlook and delete the entire contents of the FORMS folder.
The location of the forms cache depends on the operating system, where %username% is the environment variable containing the user's login name:
Windows Vista - C:\Users\%username%\AppData\Local\Microsoft\FORMS
Windows XP - C:\Documents and Settings\%username\Local Settings\Application Data\Microsoft\FORMS