So if the Solenoid is activated on shot start/finish then it isn’t necessarily part of the circuit that activates when the machine turns on and, therefore, Dave’s first thought that it was nothing to do with it might well be correct. However, as Dave adds, “you never know though”.
By the way, DavecUK, what’s an OP?
In the meantime, as extra info, I disconnected one of those white covered cables (there are two) on top of the brew boiler, the smaller boiler, and tried turning it on; it still blew. That was a suggestion from Paolo.