Uncaught SyntaxError: Unexpected token ? in JSON at position 1 is likely the cause.
It is hard to decipher the messages copy/pasted, but, this error looks like the response expected back from a batch import request was not what was expected. This indicates a server error response. You would find these under the Network tab in the Control panel. It would be near the bottom of the list on a line beginning with
admin-ajax.php (you have to have the Control panel open during the import for the network request log to record requests and responses.
If you can give me a temp WP admin login and attach a copy of your CSV file with a screenshot of the mapping you are using. I can do an import so I can see the responses. The login details can be posted here as a private reply.