@badrobot - it will install with a bit of trickery....
There will be an update to Classic Shell tomorrow that should fix the problem.
There will also be a new beta version with a ton of new features and better support for the latest Insider builds.
I just downloaded and installed the latest stable version and it worked after I changed the file name from ClassicShellSetup_4_2_5.exe to TestClassicShellSetup425.exe. But it works fine on Win10 Insider Preview Build 14342.1001.
Yup, Ivo must up his game as MS changes things - keeps him on his toes...LOL
BTW: He's done an excellent job keeping up.. can't be easy!