Our Developer got to the bottom of what happened:

The problem is a Windows update is doing this.

Access '3340 query is corrupted' errors

Günter Born first described this problem, based on reports on German-language sites, including deskmodder.de:

Microsoft Office security updates released on Patchday (November 12, 2019) cause Access to fail to access databases. An error 3340 ‘Query is corrupted’ will be dropped. …

It appears that a security update for the CVE-2019-1402 vulnerability in each version of Microsoft Office causes this error. ( https://portal.msrc.microsoft.com/en.../CVE-2019-1402 ) Here is the list of Office security updates that you can uninstall.

The following updates are those that cause the problem. Hopefully uninstalling will fix the problem until Microsoft has a permanent fix:

Office 2010: Description of the security update for Office 2010: November 12, 2019 (KB4484127)

Office 2013: Description of the security update for Office 2013: November 12, 2019 (KB4484119)

Office 2016: Description of the security update for Office 2016: November 12, 2019 (KB4484113)

At least from what I’ve seen so far, uninstalling this security update seems to allow database access again.

We are hoping that Microsoft gets an update out quickly to fix this problem.