When importing into Disqus it reports that the import had some errors. Looking at https://import.disqus.com/<disqus-id>/?a=1 one can view the import reports. In our case we have some null comments. For example
The problem is most likely with the selection of the comment content div which can in some cases contain more than a single class, eg class="commentText plaintext".
See export_comments in importer.rb.