That can also come from a sensor failure, but it takes perfectly good movies. That’s the differential diagnosis for shutter failure, because movies don’t use the shutter.
The shutter still functions, in that peering into the lens shows the shutter closing as it takes a picture, so I suspect it’s gotten a bit sticky and slow over the years. None of the various shutter-priority speeds have any effect, which means that the shutter isn’t responding properly.
A quick read of the service manual shows the Field Replaceable Unit for this situation is the entire lens assembly. Back in the day, a new lens assembly came with its own calibration constants on a floppy disk that you’d install with Casio’s service program (the latest version ran with Windows 98!) using a special USB communication mode triggered by a Vulcan Nerve Pinch on the camera. At this late date, none of that stuff remains available.
While I could take the camera apart and crack the lens capsule open, I doubt that would make it better and, in this case, ending up with a crappy camera doesn’t count for much. Extracting the lens assembly requires dismantling the entire thing, which, frankly, doesn’t seem worth the effort…
That image is number 7915: so it’s taken a bit over two images per day for the last nine years. I can’t swear the counter has never been reset, but that seems about right.
Sic transit gloria mundi, etc.