Hi,
The diagram contains pools and lanes which the Activiti Designer doesn't support.
But I also see that there's no BPMN DI information in your process XML, which is strange.
In addition there's a line with:
</extensionElements>+
And the plus sign is no valid XML. So the XML seems to be corrupt.
But you're right that the import should be more user friendly and provide a useful error message.
Best regards,