Okay, I was able to repro by creating a metric visualization with a field that is missing a type. I'm not sure how the import_dashboards script is creating such a field (this I can't repro), but I suspect this to be the issue.
Can you drill into one of the visualizations on that dashboard? I want to see if there are any errors in the edit pane, which might tell us more about which is the faulty field.
Can you also send me a screenshot of your field list sorted by type, so that fields with no type float to the top? I want to see if a field in one of those visualizations is missing it's type.
