That looks more to me like the LC is confused about what kind of monitor is attached to it than the monitor itself being broken.
Here's the pinout for the monitor end of the cable, and here's the pinout of the DA-15 end, make sure the sense lines are all carried through (a simple continuity test should do) and make sure the cable is making good contact. My theory is one might be broken, either in the cable or the monitor plug, thereby causing the LC to think it has something else attached.
(Looking at that Apple KB article the 15" Portrait needs sense 1 and 2 grounded, 0 floating. A broken Sense 2 would cause it to misidentify as an RGB 15", while a broken sense 1 would misread as a B&W two-page monitor. You could probably actually check this with a VOM yourself with the cable attached to the monitor. It's not clear to me looking at the apple article which pin should be the "sense ground"; going by the pinout of the monitor end you could noodle this out by figuring out to which pin on the DA-15 end pin 4 on the monitor end of the cable goes to. Wire up the cable and do a continuity test between whatever pin that is and pins 4, 7, and 10; 7 and 10 should be attached to that ground when the cable's plugged into the monitor, pin 4 floating.)
Here's the pinout for the monitor end of the cable, and here's the pinout of the DA-15 end, make sure the sense lines are all carried through (a simple continuity test should do) and make sure the cable is making good contact. My theory is one might be broken, either in the cable or the monitor plug, thereby causing the LC to think it has something else attached.
(Looking at that Apple KB article the 15" Portrait needs sense 1 and 2 grounded, 0 floating. A broken Sense 2 would cause it to misidentify as an RGB 15", while a broken sense 1 would misread as a B&W two-page monitor. You could probably actually check this with a VOM yourself with the cable attached to the monitor. It's not clear to me looking at the apple article which pin should be the "sense ground"; going by the pinout of the monitor end you could noodle this out by figuring out to which pin on the DA-15 end pin 4 on the monitor end of the cable goes to. Wire up the cable and do a continuity test between whatever pin that is and pins 4, 7, and 10; 7 and 10 should be attached to that ground when the cable's plugged into the monitor, pin 4 floating.)