Glad to be of help! For the purposes of completion, I'll turn my comment into an answer:
After installing Graphviz 2.44.1 for the first time on Windows, you need to run "dot -c" as administrator to complete a one-off set up.
Before this is done, if you run 'dot' at the CMD prompt, you'll get:
There is no layout engine support for "dot"
Perhaps "dot -c" needs to be run (with installer's privileges) to register the plugins?
(Which is what gave me the solution.) Running "dot -c" as administrator, creates the file C:\Program Files\Graphviz 2.44.1\bin\config6 and you're good from then on. (You can recreate the situation by deleting this file, which is how I reproduced the error text above).
@plantuml - it might be worth catching this particular GraphViz error and displaying a more helpful error message.