Using a SpinList<SpinXmlElement> as the collection of a multi instance call activity

Can you please provide a unit test that reproduces the problem? You can use the unit testing template to get started: https://github.com/camunda/camunda-engine-unittest

Also, please try to format stacktraces as code blocks, that makes them easier to read (see How to format code blocks?).

Cheers,
Thorben