I really do not know. But I have run into the exact same thing on our Route Gadget site for OCIN. I messed with it for some time and found it cleared up when I reduced the map resolution to a lower dpi. This is done by the administrator unfortunately. So you probably can't fix it. The options given by OCAD when I export the map to a JPG are 150, 200, and 300 dpi. For very large maps I have to use 150 dpi or I and many others see the red X as you did. I presume there is some kind of bandwidth crash with the Perl script that Route Gadget uses, the JPG file size, and JAVA, that sends the screen to your computer. But I am just guessing. I looked at
rg.orienteering.ca. I could see the map just fine. But I did notice it is larger than I typically have success with.
I may be barking up the wromg tree. But reducing the size has worked for me.