You can try to see if it works here: `http://f43.me/feed/test <http://f43.me/feed/test>`_ (it uses almost the same way as wallabag to retrieve content).
If it works here and not on wallabag, it means there are something internally in wallabag that breaks the parser (hard to fix).
If it doesn't works, try to extract a site config using: `http://siteconfig.fivefilters.org/ <http://siteconfig.fivefilters.org/>`_ (select which part of the content is actually the content).
Then you can submit a pull request to `https://github.com/fivefilters/ftr-site-config <https://github.com/fivefilters/ftr-site-config>`_ which is the global repo for siteconfig files.