@Frank - you can use phpmyadmin and look in the mw_list_field table and identify the field and properly update the default_value column.
You can also try setting a default value from the list custom fields area.
However, all the above will only work for frontend list display. If you need that default value in a campaign, you'd better use a filter like:
[TAGNAME:filter:defaultValue(Whatever value)]. This would make sure you don't have to update 60k rows of data but instead just use a default value if that field does not have any.