This error occurs when merging.
The merge fields being blank (not merged) is a sign that the main document has lost its link to the datasource document. When you try open the document or manually locate the datasource document (outside of the software), you will get the below "Select * from" message.
In recent versions of Word, the above message has been replaced with the below Security Warning.
If you get SQL security check message, please refer to the following Microsoft KB on how to resolve this issue:
A registry setting is required to exist for MS Word templates to be merged from a network location without a warning message appearing.
The registry setting to be added depends on the version of MS Word that is installed.
If the registry entry "SQLSecurityCheck" is missing then you need to run the associated Word20ᴺᴺSecCheck.reg file
There’s a few sources for the registry files see: Knowledge base security check
They are also shipped with the application in the application directory Insolvency\Bin\Data
Another aspect of this issue may be caused by windows security policies or profiles that cause the registry key to be removed/deleted.
Manually attaching a datasource
As a workaround or to debug the issue you can manually attach the datasource in MS Word: