I think youre right about the solenoid. We had the same issue a year ago. The fresh water pump did run every 15 minutes. For weeks we were looking for a leakage without any success. Finally we figured out that the solenoid didn´t close properly. Because we don´t use the watermaker currently I didn´t clean the solenoid but added a manual valve to stop the water flow if needed. The pic shows the valve beneath the 24V cover.
Mago del Sur - 54#40