Current a Major Bug is published up to WordPress lastest Release 4.9.6!
The real Nonsense is that the WordPress Core Team did know it since Nov. 2017 !! And changes NOTHING!
If you use the wordpress app to publish posts you must set the User Role from Author to Editor, if not you got errors to upload post updates!
Details: If you enable the basic builtin RSS Widget Module and set the RSS Feed URL like
http://www.domainname.com/feed/ the widget set the URL only on the little „RSS Icon“ but NOT on the RSS Hyperlink ! The Hyperlink is pulling the Value of $url (www.domainname.com) but not adding „/feed/“ subdir value.