Forgive me for being rude, but baratza is known for amazing service and the first thing I would do if I had a question would be to call them. In fact the FAQ in the baratza site actually describes this exact issue. {link} Check the limit switches for the interlocks first

The safety switch is on top of the boiler. It has a little red button to depress to reset

Aha! Thanks!

Maybe....but this is what the normal diffuser looks like. {image}

The machine accepts a different brass diffuser and shower screens for coffee pods. I believe you've got those installed instead. The normal brass diffuser has the holes arranged in a circle. {link}

If there is slight passing of any check valves then that high pressure gets locked in by the non relieving regulator. I assumed by the description that the pump was seeing high pressure at the intake

Isn't the expansion valve on the other side of the pump? That's why a self relieving regulator on the water supply would be the way to go. It would prevent the high pressure that would be locked in between the regulator and the pump inlet

It relieves the back pressure that gets built up during the heatup cycle, which is what you may be experiencing

Not familiar with the BST regulator but the cheaper plastic ones are usually not "self relieving" like the better brass ones like the Watts. Take a look at this thread. Perhaps the regulator is the problem. "Are water pressure regulators for RVs usable for us?"

Just hit the news this morning. Purchase price was 60$M. Should be interesting to see what happens to Baratza customer service and engineering