Update Feb 2023: This is now fixed in our latest (2023 R1) version.
In the last couple of days (Dec 2022) we have had a few reports of users experiencing similar errors when generating letters:
Last time you opened 'DATAFILE.DOC', it caused a serious error, do you still want to open it?
OR
The document 'DATAFILE.DOC' caused a serious error the last time it was opened. You may continue opening it or performing data recovery by clicking 'Recover Data'. Clicking 'Recover Data' will extract text from your document but all formatting and pictures will be lost.
You can skip past using Open/Yes options.
If the DATAFILE.DOC also gets blacklisted/disabled you may need to re-enable it via Word -> File -> Options -> Add-ins -> Manage: (choose from list) -> Disabled Items -> Go
We believe it is due to a recent Microsoft Word update but at this stage cannot pin-point where or why that is. We realise it's frustrating for users but at this stage there isn't much we can do to resolve/fix the issue.
UPDATE: You can delete the disabled items using this batch file:
REG DELETE HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Word\Resiliency\DisabledItems /va /f
