2 Jun 04, 2014 05:51
It looks like a bug when the XML file does not contain any category. Please check this page: http://b2evolution.net/man/xml-importer, there you will find a partial solution meanwhile the issue is fixed in a further release. However, please check your WP site, because at least the "Uncategorized" category should exist, so it's weird that it is not in the exported file.
If the information above doesn't help you, please enable the deugging and share any information that you consider useful in order to keep searching for the causes of your issue.