"Met.no LocationforecastLTS API states that this version of the webservice is deprecated, and the plugin needs to be adapted to use a newer version, or it will stop working within a few months. Please file a bug on https://bugzilla.xfce.org if no one else has done so yet." From the "details"-tab.
Confirmed, API version 1.3 is out and the deprecated version 1.2 will stop working in May. I will investigate what needs to be done, thanks for your report.
This should be fixed with https://git.xfce.org/panel-plugins/xfce4-weather-plugin/commit/?id=e3f53da02a6c4918c0436a8a459f81f1c9eac131. I simply bumped the version number and it seems to work fine, though there is no support for newer features (that is, the probability, though I have yet to see this in an example). A new version (0.8.9) is out already, so try this. Closing resolved fixed.
I would like to install the 0.8.8 version, but I am running Ubuntu 16.04 (xenial) and no new package is available in the xenial series.