Can someone point what's wrong

[Peter_1]

Attached, you will find a schematic of something my apprentices did some weeks ago.
This was a part of a bigger project with a lot of other regulating devices in the synoptic panel.
The 3 Eliwells (220VAC supply) are each connected to solenoid valves. There commons are linked together and switched off by the defrost clock. On the on/off switch (DPDT), they had connected the supply to the Eliwell and the supply to a small fan.
The compressor shuts off in pump down.
Defrosting was done with one channel digital clock (ventilated defrost, room temperatures +/- 2°C)
Something very strange happened when there a defrost started .
What?
They searched almost two hours to locate the problem thinking that those who had layed the multicables (this was not done by us) to the different devices (sometimes far awy) had switched the connections, that they even had switched the cables (numbers on the cables fault?), lose connections, wrong supply voltage ....
They called me and als i didn't saw the fault just after one of them said something - on the 1st hand very stupid - which was the origin of the problem.
What?
See it as a sort of a technical riddle.