Hi Maren,
As far as I understood OpenJUMP 2+ has some signficiant changes (e.g. an updated geometry library) and its very likely that there are incompatibilities which will not load the toolbox (and some functions may not work).
Hence, i recommend to download the - a bit old - package of Horae with OpenJUMP 1.x, e.g. from ResearchGate:
I hope it still works on newer Java Versions. If not please let me know (though no promises here as I am not having a developer setup on my laptop since a while now).
Best,
Stefan