I'm working on a fairly simple PCB that interconnects three arduinos. There are also MOSFETs for switching some solenoids. The system runs off of a 24V DC power supply. I was running a LM7812 voltage regulator, but I added a pair of cooling fans and they draw about an amp combined so I switched out the LM7812 for a R-78B12-2.0 DC-DC regulator.
I dropped in the 78B12 and everything looked fine. I ran the pair of fans for a couple of hours and everything seemed good. Nothing was warm, voltages were stable. I dropped in my arduinos and fired it back up and clearly something went wrong.
The trace from from 3.3uF input cap on the 78B12 to ground was smoked and the 78B12 is now shorting 24V straight out. That blew up one of my Arduinos and both of my cooling fans.
I'm not sure where to start to diagnose what happened and would appreciate the help.
EDIT: Here are my schematics (with fixed ground on U6 and correct C1)