Thanks a lot, that did indeed solve some of our problems. However – even though we closely followed the instructions – one issue remains: It is not possible to select multiple Energy Sources or Projects, SF does not return the correct result. We already tried the approach described as “Caveats” in the documentation, but unfortunately, the functionality resulting from this implementation is not what we try to achieve, because then, after applying one filter, not all of the checkboxes are displayed anymore. The user of our site should be able to select multiple Projects/Energy Sources by ticking multiple checkboxes.

I have tried several times to post another answer on this topic, but unfortunately it does not show up when I click on the “Submit” button, but disappears and I get this message:
“ERROR: Duplicate reply detected; it looks as though you’ve already said that.”
Maybe it depends on the length of the answer because my short answer yesterday was posted.