Hi again, took a break over the weekend, came back this morning and did some poking at this issue.
Turns out this was my fault, not Ansible’s. The XML error was a red herring - the real issue was RPCError is not defined. I didn’t have ncclient installed, so an import statement in the netconf module was failing, which caused the entire task to fall over. 
If you’re running into this issue, make sure you’ve got ncclient installed as a system module. On Ubuntu and Debian, this is python3-ncclient.