That's the good news. The bad news is that we discovered shortly after that Asciidoctor 1.5.6 is not compatible with Asciidoctor Diagram 1.5.4. We'll need a new release of Asciidoctor Diagram. We've sorted out a fix and Pepijn said he will be putting out a new release soon for you to use.
To avoid this problem in the future, we're setting up a build pipeline for the ecosystem that gets triggered each time a change goes into master. So far, Asciidoctor.js and AsciidoctorJ are enlisted in this pipeline, but obviously we'll want to get Asciidoctor Diagram in there as well.
So spread the word, if you upgrade to Asciidoctor 1.5.6 and you're using Asciidoctor Diagram, you'll also need to upgrade Asciidoctor Diagram (likely 1.5.5).