Hello,
I have an exception (ClassCastException) when using gradient in my skinparam definition and only in the sequence diagrams (net.sourceforge.plantuml.graphic.HtmlColorGradient cannot be cast to net.sourceforge.plantuml.graphic.HtmlColorSimple).
If the format is PNG, this is working fine (but I need SVG to have hyperlinks in the generated diagrams).
I was with version 1.2017.16, and tried also the 1.2017.18beta22 (from a previous similar Q&A), with the same behaviour.
Thanks and regards.