Warning … Unavailable

On some, but not all of my website pages, I’m seeing a yellow box that says “Warning: http://www.xxx.com/?p=2304 is unreachable.”

Everything seems to be working fine. It’s even possible to post a comment. But the error message is annoying and likely to confuse site users. How can I fix this? Thanks.

https://ww.wp.xz.cn/plugins/facebook-comments-plugin/