I am seeing the same message with Arbortext Editor 7.0 on Windows 7. Our users still on Editor 5.4 are not reporting this issue, so I assume that it is not a Windows 7 problem.
I am guessing that you are using something like this in the doctype .dcf config file:
<ActiveXControl element="date" controlName="date" scriptFileName="date.js" scriptName="date" programId="MSCAL.Calendar" eventName="DOMActivate">
Anybody have any suggestions?