Here is a great layman's explanation for back emf diodes (the diodes directly mounted to most solenoids).

The video is covering a DC motor but it basically operates on the same principle. As far as the circuit is concerned, it doesn't know or care what the actual inductor is - either motor, relay or solenoid.

Note what happens to the controlling transistor's input voltage at 1:40 when there is no back-emf diode installed.
And - at 4:00 --> this may be what is happening to the 48V and cooking the new stern node boards.

I wish I created this video but I should have noted that this video was made by somebody else.
To give full credit to the creator -- this was made by a person with the ID "Afrotechmods"
https://www.youtube.com/channel/UCosnWgi3eorc1klEQ8pIgJQ