Water leaks onto the internal cpu board and fries it. La Marzocco apparently did not put a waterproof coating, cover over it, or move it elsewhere, so your only option is to buy a new one (for $500), disassemble your whole machine, fix it and reassemble the whole thing. The vacuum breakers malfunctioning is another common complaint. They supposedly found a way to fix it on the new ones, but things as simple as 'put a waterproof cover / coating over the board or move it' should have already been thought of in a machine costing as much as that one.
The owners of the GS/3s on HB consistently have new problems, which would be the last thing I would expect at that price point. It just seems like they put a lot of R&D into the espresso making design, but not the practical, everyday use machine design.