I took a look and their new post seems to have at least one invisible, invalid character which breaks their feed:
Previously, I manually edited over a dozen posts to find and delete all those characters in order to fix their feed. However, it's not feasible for us to edit every new post this particular user makes in order for their posts to show up in the Reader, especially when they themselves aren't contacting us regarding the problem.
Since other Korean blogs show up fine in your Reader, the problem points to how this user writes their posts and most likely is due to composing in an external word processing program and copy/pasting the text into the Post editor.
You might want to contact the blog owner to let them know that there is an issue, so they can try another method for bringing their posts into their blog. You can also let them know that they are welcome to contact us directly if they would like assistance with this issue.