(REPOST FROM MY OLD BLOG) Occasionally when working in SharePoint Designer, I received the following error occurs when trying to Check In or Check Out files.
For me the problem was very frustrating because it appeared to be intermittent. I did some searching online and tried many suggestions but still continued to receive the error. Finally one day when I was looking at the top title bar in SharePoint Designer, I noticed that I was using http:// instead of https:// for a secured site. I closed my site and re-opened it using https://. Amazingly, Check In and Check Out now worked great and I no longer received the error message!
