Jochen Kalmbach was kind enough (thanks Jochen!) to point in a comment to my previous post that release 6.6.3.5 is a BETA version of WinDbg.
He also pointed out this post in the WinDbg newsgroup that confirms this information.

Sorry for not finding this out just now, but it seems the only place that the word “beta” was written was in the “redist.txt” file in the installation folder.

It seems that some code Jochen wrote doesn’t work with the new version of DbgHelp.dll so there are a few things you should watch out in betas…
I personally like to live on the bleeding edge when it comes to WinDbg so I usually do care if a version is a beta or not. On the other hand, I can sympathize with Jochen’s frustration about using a new beta version and finding out something is broken…

Oh well… I’m sure this will get fixed very soon.

I just wish WinDbg releases were a bit more often than every 6 to 9 months.