In any event, observe the voltage across the RCD diode to see if this might be the culprit.
I have. The diode is rated to 1200V and parallel to a 100 Ohm (pulse) resistor. I don't see how it would even be possible to get 1200V out of the system, given the 300V input. There are RC, RCD snubbers and protection diodes. It's all very depressing.
Hmm, it wasn't made clear that this is an outlier among a number of units that have been working for a year; that does rather complicate things.
That said, I'm still leaning towards the instantaneous dissipation during forward recovery being too high - see Fig. 6 in the diode datasheet which plots t
fr and V
fr vs. dI/dt. For example, if current is changing at ~300A/us then tfr will be ~500ns and Vfr will be ~20V... Needless to say, that's quite a bit more dissipation than might naively be assumed from the conventional forward voltage spec!
Ironically (or perversely, really), "fast" diodes often have a slower forward recovery time compared to their reverse recovery, and in some cases a standard recovery diode will outperform the fast type in forward recovery! As you have configured the snubber to limit dV/dt (R || D), rather than as a voltage clamp (R || C), you might want to try both standard recovery and SiC Schottky diodes here.
Otherwise, you may end up needing to use a higher current diode, and/or multiple diodes in parallel (with a ballast resistor for each of them).