I believe the CUI command is .NET based, as are most of the functions in ToolPac. Perhaps your .NET Framework 4 is corrupt and needs to be reinstalled?
Having recently re-installed both the previous release of AutoCAD 2012 as well as your excellent ToolPac utilities, I had no such errors nor crashes result from neither the CUI manipulation nor from invoking any ToolPac commands.
It appears that there may be other causes that affecting the stability of the 2013 release.
Both AutoCAD 2013 and ToolPac 14.1 running on it uses Framework 4 as opposed to previous versions using previous versions of the framework. So a corrupt Framework 4 wouldn't have affected 2012/TP13, etc.
POSSIBLE CAUSE FOR ISSUE:
Prior to installing Product Design Suite Standard 2013, I realized that the new Visual Studio Beta version was installed and may have contributed to the instability with AutoCAD 2013. I have now uninstalled the VS Beta.
I have been encouraged by the timeliness and substances of the responses from Autodesk support staff and other developers and users here.
Due to the excellent workarounds and responses given, I am now feeling comfortable enough to re-install AutoCAD 2013 and, as time allows, run the program.
However, AutoCAD 2012 and other 2012 products will be used for production purposes at this time.