Identifying unhandled interrupt
You can add a new handler with an infinite loop one at a time until the missing IRQ handler is detected.  It's a slow, but reliable method.  You can find the full list of weak IRQ definitions in the startup_stm32.s file.

Example incremental IRQ handler addition to stm32f3xx_it.c:

void DMA1_CH1_IRQHandler(void) {
  while (1);
}


In general, this seems to be an issue of unknown defaults.  Starting a project with all the IRQ handlers implemented is one way to make sure you're not bitten.