After upgrading to the latest launcher, it no longer find the java runtime. If I delete the existing copy of the java runtime in my minecraft directory, it happily downloads a new fresh copy, but upon trying to launch any profile it comes up with an error message stating

Unable to locate the Java runtime.
Error details: The filename, directory name, or volume label syntax is incorrect
Filename on disk: javaw.exe
Path: C:\Games\SSD\Minecraft\runtime\jre-x64\1.8.0_25\bin\javaw.exe
Exists: file

The directory is there, the expected files are there, and it worked perfectly fine under the old launcher.

So, the new launcher knows how to download the java runtime, it knows where to put it, it knows where it is, but it still fails to use it.

The version of java is the exact same as is used by my curse-managed modded minecraft installs, which still use the old launcher, and there everything works flawlessly still.

Note that manually changing the java version used by the profile from the default of '<use bundled java runtime>' to use my normal java install also works fine, the problem seems to be with the launcher using the one it manages itself.