This error is quite annoying for some time, because there is no actual xml file in WordPress that handles the RSS Feeds. I tried to search through Google and found out some links showing solution to this problem. But none of them works fine to my situation. Here are those links:
The reason that these links could not be helpful is, I guess, WordPress is improving the RSS Feed links in its newer version. But this problem somehow got me again. The only thing I did, in this case, is to get rid of additional “blank lines” at the end of functions.php in my selected theme folder; and made sure that php closing tag is attached to the end of functions.php file without any more blank lines left after the closing tag. Thus, XML declaration was moved to the very top of rendered xml document by WordPress; and then WordPress default RSS Feed link was ready to work.
Therefore, my suggestion when encountering this kind of similar problem in WordPress is to check all PHP files in selected theme folder first to make sure all php files are closed properly with closing tag “?>” and do not leave blank lines after closing tag. After this is done, if problem still presents itself, then we could try the above three links.