I think this is because of :
==> Database contains an entry for a plugin and files for that plugin does not exists, when joomla goes to require those files then found cannot find any file for that plugin.
What you can do:
==> You have to go to the line where joomla requires files (as you have mentioned above "JPluginHelper::_import") and then Print the path. By doing this you will see all required paths(in browser) and last path you will see the path which is rising error.
====> Check that file exists or not.
====> Now find entry for that plugin in extensions table and set published to '0'.