Exp-00091 Exporting questionable statistics.

Set the statistics=none to avoid this error.
The default value is Estimate.

The error is caused by one of the following

1. Your table rows have fatel error while exporting.
2. You have used query parameter
3. client and server characterset mismatch.

After importing the data you have to gather fresh statistics to use it. So it is always advisable to use statistics=none.