Hi Bruce, thank you for your help. Indeed, loading both functionalities (require 'oc-biblatex) (require 'oc-natbib) did the trick. That's strange, though: my version of Org (get from M-x org-version) is Org mode version 9.5.3 (9.5.3-g69c588 @ /home/alessandro/.emacs.d/elpa/org-9.5.3/). Shouldn't it already be fixed?
Moreover, I now encounter a different problem: biblatex works great, but natbib doesn't. It's not a big problem, since I use biblatex, but here it is, if someone is interested. If I set up natbib as reference engine (with #+cite_export: natbib kluwer), and I export to a pdf file (C-c C-e l o) the references are not resolved, i.e. they are shown as "(?)". However, if I export to a LaTeX file (C-c C-e l l), and then compile it, everyting is fine (although it displays some annoing "n.d."). After that, if I try again to directly export to pdf, it works. I suppose it's a problem in producing the intermediate bibliography files. Alessandro
