In T54253#573642 Nemo_bis commented
Maybe worth mentioning, due to protocol-relative URLs our feeds do not validate: "id must be a full and valid URL", e.g. http://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fwww.mediawiki.org%2Fw%2Findex.php%3Ftitle%3DSpecial%3ARecentChanges%26feed%3Datom%26limit%3D50
The W3C help for the error says "If this is a link to a web page, make sure to include the "http://" at the beginning."
The entry's <link rel="alternate" href does contain the full https URL, e.g.:
<entry> <id>//www.mediawiki.org/w/index.php?title=Project:Language_policy/ps&diff=1415363&oldid=1415356</id> <title>Project:Language policy/ps</title> <link rel="alternate" type="text/html" href="https://www.mediawiki.org/w/index.php?title=Project:Language_policy/ps&diff=1415363&oldid=1415356"/> <updated>2015-02-19T18:20:35Z</updated> ... <author><name>ANBI</name></author> </entry>