I did not find any information on this, so I am going to post it here.

Selfoss is using graby for full text content retrieval. I noticed that some of the newsfeeds I read stopped working.
In my case, the reason was a change in the page source which needed adaption of the graby site-specific config files which are kept in "vendor/j0k3r/graby-site-config/[domainname].txt"

To find out the necessary changes, you could use this tool:
http://siteconfig.fivefilters.org/