I'm guessing tinyMCE itself is changing the URLs to be relative to the page you edited them on. I'd try looking for an option to use absolute links. If you aren't having luck you can try this hack to just get something going.
I'm using tinymce with a custom build link and image plugin. I now can upload files and images and it is possible to insert them into the text_area. The urls look like /documents/id/name. But when I save the entries in my db, the url changes to ../../documents/id/name.
Is there a way to prevent rails form changing the url?