Checkin fails with error message complaining about missing DTD (1003)

Symptom

On checkin or import operations on DITA content, Cinnamon Desktop Client reports a missing DTD.

Background

DITA files, even though when based on an XML Schema, need an XML Doctype Declaration, otherwise DITA editors and DITA-OT don't function correctly. This declaration is misinterpreted by legacy Microsoft XML libraries in .net, trying to load the DTD which is not actually present (and not needed).

Solution

The issue manifests itself in .net 4 and is fixed in newer .net releases. Upgrading to a newer .net release (like 4.7.2) fixes the problem.