Here is the solution:
First edit the example.html (change to the correct paths inside, see above) and then copy it into both folders:
/etc/openhab2/html/weather-data/layouts/
and
/var/lib/openhab2/webapps/weather-data/layouts/
Or make a symbolic link (symlink) to example.html (to /etc/openhab2/html/weather-data/layouts/example.html)
IMPORTANT: the paths inside example.html always have to begin with: static/weather-data/…
The string ‘static’ at the beginning of each path causes openhab2 to search in the folder /etc/openhab2/html/