When updating NCrunch from 3.7.0.7 to 3.11.0.9, on Visual Studio 2017 Community 15.4.3, the installer put it to:
C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\Common7\IDE\Extensions
Of course Visual Studio didn't find it there, so everything went wrong (extension could not be found, activitylog.xml refers to old directory from 3.7.0.7, followed the instructions in the forum including devenv.exe /setup, now everything's gone, no ncrunch menu at all, and so on). Only a manual installation to
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\IDE\Extensions
could fix it. So everything's fine now, for me the problem is solved, but I'm not sure why this happened and whether the installer should be fixed.