When importing content to a new Drupal 9 site from a Drupal 8 site (which had previously been migrated from D7), I get this error which halts the process:
Symfony\Component\Serializer\Exception\UnexpectedValueException: "comment_node_article" is not a valid bundle type for denormalization. in Drupal\serialization\Normalizer\EntityNormalizer->extractBundleData() (line 119 of /[my site root]/core/modules/serialization/src/Normalizer/FieldableEntityNormalizerTrait.php).
I had been able to work around some other errors about some node types not existing, but I don't know how to address this one. Besides, of course, simply removing the comment items from the data to import and then maybe pasting them in later?
Content import from D8.9.20 to D9.4.0 using Content Synchronization 3.0.0-beta1 with content of type "comment.comment_node_article" in the data to be imported.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.