Images, that have been uploaded to a page with such a page title, can not be used on other pages.
Example: We have a page called "1.)Test" with an image attached. If we now try to use this image at another page (e.g. using the image browser and searching for the image) "unknown attachment" is shown at this page. In the editor first everything seems to be fine, be after saving the page, this information appears.
After we remove "1.)" from the page title and try to use the image again, everything works fine.