"Unable to read message from message file" indicates the corresponding string for the label was not found in either DCADWIN.lbl, DCADWIN3.lbl, DCADWIN.msg, DCADWIN3.msg, or DCADWIN.mnu. This usually occurs when new lines are missing from the end of these files. Make sure you have the latest version by going to Help, Check for Update...
As Roger indicated, you can also try re-running the installer, DataCAD18Setup.exe, and performing a Repair install if the latest version is already present.
Also make sure that in the Pathnames tab of the program prefs that the Messages and Labels path matches where the files that Mark mentioned are located for your current version, typically it's Support Files\Messages and Labels\
Thanks! - Josh Do. Or do not. There is no try. Josh's Digital Downloads is come back online soon. Stay tuned. (soon is a relative term)