How to resolve an XMLReader warning produced by Wordpress Migrate module?
I had the exact same error.  I uninstalled all the WordPress migrate and migrate modules (uninstalling migrate may not have been necessary), removed from site directory and then reinstalled. 

I re-installed Migrate 7.x-2.6 and WordPress Migrate 7.x-2.3-beta2+4-dev.  

After that the import worked.