The current Kdenlive on windows (September 2018, version 18.08.1) has a few issues that have workarounds. The purpose of this wiki page is to document these issues and their workarounds.
Currently, if you use any method from playing the preview on Kdenlive for windows, the preview of your project will not play. This includes trying to use the spacebar, or trying to press the play button using your mouse.
The quickest and easiest workaround is to use the Spacebar + Right arrow key. Press the spacebar and then press the right arrow key.
Another option is to change the settings. To do this, go to Settings > Configure Kdenlive > Environment and change the process threads to 2 or above.
When switching from kdenlive for windows 17.12 --> 18.04/18.08, a QT rendering crash appears. To make sure this doesn't happen, you need to edit the kdenliverc file in the appdata. To access your appdata, press Win + R (Windows key and R key simaltenously) and type APPDATA (all caps). Find the kdenlive folder, and within it rename kdenliverc to kdenliverc.old.
This describes the process of doing a clean install on windows.
Firstly, delete your normal kdenlive folder (containing the application)
Access the Appdata folder (WIn + R and then type APPDATA in full caps). If you have any saved effects or clips stored in your library, make a backup of the library folder. Then once you have backup up your library folder, delete the kdenlive folder.
Reinstall the latest version of kdenlive from https://kdenlive.org
If you want to reinstall kdenlive, it may tell you "The file or folder is open in another program". Windows then won't let you delete kdenlive.
To fix this, press and hold Ctrl + Shift + Esc & expand the task manager by clicking all details. Then find kdenlive.exe & dbus-daemon.exe, and click End task for both of them.
This bug can appear if you do a clean reinstall of kdenlive (see above). Simply close and open kdenlive once, and it should be fixed.