The first thing to do is to temporarily disable all other plugins installed on your WordPress and restart the import. This is to avoid conflicts with some plugins that might interfere. If this is successful, then remember to re-enable all your plugins.
If this fails, it is probably corrupted data in your database.
Then start by checking that the logs are enabled in the 'Plugin settings' tab of the GraphComment plugin. Then restart the import and after the failure, copy and paste the logs in an email you send us at firstname.lastname@example.org.