The update code that drives the automatic updates was changed at the end of last week, and unfortunately this broke the update mechanism on Windows.
If you downloaded a trunk nightly build on Saturday or Sunday, then you may be affected. You’ll know this is the case if you get a dialog stating “The update could not be installed (patch apply failed)”.
The cause has now been fixed and was included in Monday’s nightly builds.
If this affects you, you will need to download and install a new nightly build from one of these locations: