You can try to see if it works here: `http://f43.me/feed/test <http://f43.me/feed/test>`_ (it uses almost the same system as wallabag to retrieve content).
If it works here and not on wallabag, it means there is something internally in wallabag that breaks the parser (hard to fix: please open an issue about it).
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). You can `read this documentation before <http://help.fivefilters.org/customer/en/portal/articles/223153-site-patterns>`_.
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.