asciidoctor-diagram: ditaa and plantuml error

asciidoctor-diagram 1.5.1 Asciidoctor 1.5.4 Ruby 2.2.5 on 64-bit Cygwin

ditaa and plantuml diagram generate an error: asciidoctor-diagram: ERROR: Failed to generate image: invalid byte sequence in UTF-8.

For instance

[ditaa]
----
                   +-------------+
                   | Asciidoctor |-------+
                   |  Diagram    |       |
                   +-------------+       | PNG out
                       ^                 |
                       | ditaa in        |
                       |                 v
 +--------+   +--------+----+    /----------------\
 |        | --+ Asciidoctor +--> |                |
 |  Text  |   +-------------+    |Beautiful output|
 |Document|   |   !magic!   |    |                |
 |     {d}|   |             |    |                |
 +---+----+   +-------------+    \----------------/
     :                                   ^
     |          Lots of work             |
     +-----------------------------------+
----

or

[plantuml]
--
class BlockProcessor
class PlantUmlBlock
BlockProcessor <|-- PlantUmlBlock
--