I could finally reproduce this bug on a clean installation. This bug happens because the clock applet sends the system timezone to libgweather and the timezone identifier is considered "obsolete" by libgweather. (See bug https://bugs.launchpad.net/ubuntu/+source/libgweather/+bug/291853 about obsolete timezones.) Setting the location from the clock applet (using the Set... button) or changing the system timezone is a work around for this bug.
I am marking this bug as Confirmed and sending the bug upstream.