Just letting you know about an error I found (and solved):
I was importing a CSV file with only 10 rows. Whichever fields I chose to import I got the same error:
"1 posts with no valid UserID/Name are assigned admin as author."
The first row in my CSV file was imported with Draft status. All other rows were imported as Published posts. Most fields were empty.
I figured out that if I didn't use the drop-downs and instead, typed the custom field names in manually, it worked ok.