Follow these steps to resolve the issue:
Close all Revit Sessions on the affected machine.
Rename the following file (instead of removing the folders listed in the solution below): C:\Users\%USERNAME%\AppData\Local\Autodesk\Revit\Autodesk Revit %VERSION%\DDGraph
Restart Revit.
Alternatively:
Copy and paste a healthy DDGraph file from another machine, where the issue is not reproducible.
If the above steps do not help, clear the content of the following folders:
C:\Users\%USERNAME%\AppData\Roaming\Autodesk\Revit\Autodesk Revit %VERSION%
C:\Users\%USERNAME%\AppData\Local\Autodesk\Revit\Autodesk Revit %VERSION%
%USERNAME% should be your windows login name.
%VERSION% should be your Revit version.
