This error message occurs when actions are performed while the system generates the mail merge template. The template needs to be fully generated before any more actions are performed on the computer.
Create a new mail merge using the document type Word 97-2003 instead of Word 2007/2010.
You can follow the steps below to achieve this.
In the Admin Tool, click Mail Merges.
Click Create a new mail merge.
Use the Type drop-down and select the document type Word 97-2003.
Create the Data Source.
When you click on View/Edit Template, this will start creating the template with all of the Merge Fields included.
At this stage, do not click anything on your keyboard or mouse while the merge template is generating. This will cause the template to get interrupted, and it will fail with the error message - Exception has been thrown by the target of an invocation.