Red x’s indicate that a file used for a view image or conditional format icon is missing. A file that is removed, moved, or renamed is considered a missing file. Normally, a red x would display in place of missing files; however, in some cases you may see a blank box instead.
Blank boxes and red x’s can be corrected for conditional formats by replacing missing files or updating the conditional format. Blank boxes and red x’s can be corrected for images by replacing the missing file or updating the measure item expression.
See also: Why do Blank Boxes Show in Views or Conditional Format Windows?
Here is a view with a calculated measure item that displays product images.
The files for the product images are moved, resulting in red x’s the next time the user runs the view.
The user updates the calculated measure item expression to reflect the new location for the image files. The refreshed view displays the desired product images once again.