ok, found the very strange reason why it didn't work.
Before editing the shadow4 skin directly I tried the script in my skin. Of course it didn't work correctly since the call for the script in weather.xml goes out to the shadow4 skin lua file (I only noticed that later). So I removed the not working "calendar" in the skin settings from the active calendars and added the shadow4 weather calendar.
But for some strange reason Rainlendar still used the weather.lua from my skin. So no matter what I changed, it always used the wrong weather.lua file. Very strange why Rainlendar would do that and I'm very annoyed that I spent two hours or so figuring out what goes wrong in the script only to find that Rainlendar used the wrong one
Maybe the log needs to track down the path of the script used from skins, too.
Also it is strange that even though I changed the url of the Shadow4 Weather skin but it used the wrong lua file....