This indicates a problem with the monochromator, often caused by a failed lamp, or the optical fibres. you can check if it is a lamp by doing this: in Unicorn, go into manual controls> Flowpath. Select injection valve and load. This is a quick way to activate the UV-900, there are other ways to activate the UV-900, but this one way to run it without activating the pump. While the UV-900 is on place your ear (it doesn't matter which one, as long as it is a functioning ear) next to the side of the UV-900 without the flowcell on it. you should here to noises. one is similar to a faint cricket sounding buzz, and an electric motor that is running and stopping repeatedly. If you do not hear the crickets, the lamp has failed and needs replacement, if you do not hear the motor sounds, or it sounds loud and expensive, it needs to be replaced or worked on.
The other possibility is a problem with the optical pathway, fibers, grating, lens etc. This will require a repair.