I'm seeing a similar, possibly releated, problem with v0.9.42. Everything that is failing in my 0.9.42 installation worked in my previous v0.9.38 installation.
I'm running 32bit Calibre on 64bit Win8. At the system level, my default app for .MOBI files is Kindle Previewer. Calibre is running well and accesses my library (2 folders down inside of My Documents) to add and convert books.
What fails is Calibre's access to the books when clicking on the format in the Book Details panel (no response of any sort when it should be passing the book to the system's default handler which is Kindle Preview in my case). Also the Open With pluging fails with an error message:
calibre, version 0.9.42
ERROR: Unhandled exception: <b>error</b>: (2, 'CreateProcess', 'The system cannot find the file specified.')
Traceback (most recent call last):
File "calibre_plugins.open_with.action", line 178, in open_with
File "calibre_plugins.open_with.action", line 221, in launch_app
error: (2, 'CreateProcess', 'The system cannot find the file specified.')
The Open Folder Containing function opens the proper book folder in an Explorer window and double clicking the book opens Kindle Previewer properly, which proves the system level association of MOBI with Kindle Previewer is correctly set.