The question regarding the exclusive use of the 24v power bus only for this purpose of supplying the LED's arose due to this voltage level is usually used in industrial automation, where motors or inductive drive devices are commonly used ( e.g solenoids, contactors ), and another possibility could be the passage of this 80m cabling (bifilar? shielded?) near to some inductive EMI source, such as fluorescent lamp circuits... whatever. Have you tried to replace this 1uF capacitor with another one with a nominal voltage above 50v?
