Changing Category Name breaks RSS feed (does not validate)

We recently changed a category from “Messages” to “Blog” but the RSS feed for http://news.feedsy.info/category/blog/feed does not validate here: http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fnews.feedsy.info%2Fcategory%2Fblog%2Ffeed

However the old category URL does validate (even though it is now empty of content): http://news.feedsy.info/category/messages/feed

The URLs of other categories also validate.

Note: We changed Category by Dashboard > Posts/Categories > Edit “Messages” and changed the Name and the Slug to “Blog” and “blog”.

The errors were:

line 1059, column 1: Undefined item element: br (28 occurrences) [help]

line 1060, column 0: Undefined item element: b (42 occurrences) [help]
<b>Warning</b>: imagecreatefromjpeg(): Filename cannot be empty in <b>/home …

line 1060, column 14: Unexpected Text (29 occurrences) [help]
<b>Warning</b>: imagecreatefromjpeg(): Filename cannot be empty in <b>/home …