The LDR composant (photoconductive cell) is oxydated.
I would change it.
Can you tell me the characteristics of this composant ?
Check the dimensions on the Velleman site of these two possibilities: LDR09 and LDR04. I believe it is the LDR04. But again: check the dimensions on the Velleman site against these you can measure with your defective component.
LDR04 will do the job.
Basically, almost any LDR will do the job.