Asciidoctor Diagram 1.5.5 is now available. You will need to use this version if you want to use Asciidoctor Diagram with Asciidoctor 1.5.6.
https://rubygems.org/gems/asciidoctor-diagramThanks to Pepijn for turning this release around so quickly, and to Robert for helping to identify the problem and the fix.
Cheers,
-Dan
p.s. Robert is working on the AsciidoctorJ 1.5.6 release. In fact, this problem was detected during QA while preparing that release.