Steps to reproduce:
Create a Cascade Select Field
Create a Multiple Select Field
Create a post-function that uses Copy Value from Other Field, from field 1 to 2.
This error occurs:
I saw other problematic behaviors anytime you try to do anything with any cascade select field.
Then, I found this very reasonable explanation: https://answers.atlassian.com/questions/103164/getting-java-lang-classcastexception-java-util-hashmap-cannot-be-cast-in-jira5
So maybe it's time to review the source to use the right class when dealing with this type of field.