D7->D8 upgrade fails with "missing bundle for entity type node"

When I try to upgrade a site through the UI, with about a couple hundred nodes and half a dozen users, the module returns a success message but fails in the background. The system log returns "Upgraded ________ (processed 0 items total)" for each module, returning zero even for those that have many records. After that, it returns "Source ID ____: Missing bundle for entity type node" for every node that I have. In the end, it upgraded not a single content node, user, content type, taxonomy vocabulary or term, nothing at all. Also, it says in the beginning that Book has no upgrade path, even though the module is enable both on the source and destination sites. I followed the UI upgrade instructions on the Drupal website.