I recently encountered this problem after upgrading to the latest version of WordPress. This was not the first time I came across this problem. Earlier when feedburner owned the service, feedburner staff solved the issue for me. But ever since Google has acquired, the support has been not worth mentioning.
However, I found two ways to fix this problem:
1> If you understand the internal structure of XML and have some programming background, then you can validate your XML at http://www.feedvalidator.org/ for free. This site will point out the problem and also will tell what you need to do in order to fix the problem.
2> If you have absolutely no idea about programming then you can download and use the plugin – ‘Fix RSS feed’ from here, which will solve this problem.