asciidoctor-diagram: Asciidoctor-Diagram fails on JRuby

I just tried to build a new AsciidoctorJ diagram. But it doesn’t generate a diagram and just emits this message:

asciidoctor-diagram: ERROR: Failed to generate image: Ditaa image generation failed: Invalid path '/ditaa'

Same for plantuml.

I couldn’t reproduce it running the pure Ruby version. I also got it working if I change java.rb to always behave like running on a native Ruby:

#if RUBY_PLATFORM == "java"
#  require_relative 'java_jruby'
#else
  require_relative 'java_socket'
#end