Yikes, that’s an ugly error message - you might want to contact Salesforce Support with the content of that error message. Do you see the Send Email action on standard Case layout view?
I saw this about having to have Lightning Experience enabled in order to use Send Email, but I’m not sure if that’s really true: http://resources.docs.salesforce.com/200/7/en-us/sfdc/pdf/actions_impl_guide.pdf
