xfce4-weather-plugin 0.8.10 says in German:
Die met.no-LocationforecastLTS-Schnittstelle hat mitgeteilt, dass diese Version
des Netzdienstes veraltet ist. Wenn die Erweiterung nicht an die neue Version
angepasst wird, wird sie in einigen Monaten nicht mehr funktionieren.
Bitte erstellen Sie eine Fehlermeldung auf https://bugzilla.xfce.org, wenn das
noch niemand getan hat.

Google Translator:
The met.no-LocationforecastLTS interface has announced that this version
network service is out of date. If the extension does not match the new version
adjusted, it will no longer work in a few months.
Please create an error message on https://bugzilla.xfce.org if that
nobody has done yet.

A bit more detail.  met.no published a new API for getting sunrise/sunset information.  Version 2.0 appears to have completely superseded version 1.1.  Documentation on the new API is here:  https://api.met.no/weatherapi/sunrise/2.0/documentation

Old format: https://api.met.no/weatherapi/sunrise/1.1/?lat=40.7127;lon=-74.0059;from=2020-03-06;to=2020-03-08

New format: https://api.met.no/weatherapi/sunrise/2.0/?lat=40.7127&lon=-74.0059&date=2020-03-10&offset=-05:00 

The fix may not be as easy as fixing the API call,  The output data also has changed.

I also get the API warning.

	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.

Comment 4 Harald Judt 2020-04-05 23:53:12 CEST

*** Bug 16596 has been marked as a duplicate of this bug. ***

Comment 5 Harald Judt 2020-04-05 23:56:17 CEST

Seems to be fixed in the latest version, so why not use it instead of the old one?

I'd wager the answer is simply because the latest version is not what is in the repo's for our given distro's.

xfce4-weather-plugin 0.8.11 here