I found some more exceptions with gradients. It might be the same problem as here:
http://plantuml.sourceforge.net/qa/?qa=6481/exceptions-during-export-with-color-gradients
This time, the exception message is inserted into the exported file. The export doesn't fail.
The result looks like this:
\node at (5pt,5pt)[below right,color=black]{An error has occured : java.lang.ClassCastException: net.sourceforge.plantuml.graphic.HtmlColorGradient cannot be cast to net.sourceforge.plantuml.graphic.HtmlColorSimple};
\node at (5pt,20.2109pt)[below right,color=black]{If I cannot bring you comfort then at least I bring you hope};
\node at (5pt,35.4219pt)[below right,color=black]{ };
\node at (5pt,50.6328pt)[below right,color=black]{Diagram size: 17 lines / 168 characters.};
\node at (5pt,65.8438pt)[below right,color=black]{ };
\node at (5pt,81.0547pt)[below right,color=black]{PlantUML (1.2017.19beta3) cannot parse result from dot/GraphViz.};
\node at (5pt,96.2656pt)[below right,color=black]{ };
....
The exception was created with this diagram:
@startuml
skinparam package{
BackgroundColor red-green
packageStyle Rectangle
}
namespace DefaultNamespace {
}
@enduml
