All versions of Office are not compatible with each other. For example, if a user on Office 2007 creates a conditional or simple mail merge, users on Office 2003 may receive errors when using the merge.
Try each of the following to resolve the issue:
Click Save and Return to EE located under Add-ins in the Word Merge document
Install Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats.
Update all workstations to Microsoft Office 2007.
Note: We provide links to third-party websites in an effort to help you resolve your issue. We are not responsible for the information on third-party websites, and we cannot assist with implementing resolutions from these websites.