Error 304 means the BlueBox fails to connect and establish debug session with the Cortex-M-based target CPU.
When troubleshooting the initial debug connection to the target CPU, it's recommended to use the CPU Reset debug command instead of the Download debug command.
Check if the BlueBox is properly connected to the target debug connector
through the usage of proper BlueBox Debug Adapter (iC5000, iC5700).
Double-check that the pinout of the target debug connector matches with the
one from the debug adapter. Pinouts can be found in the
Debug Adapters User Manual.
If a wrong debug adapter is used, the debug connection to the target CPU will fail and in the worst case the hardware can get damaged too.
Cortex-M-based target typically provides 10-pin or 20-pin target debug connector featuring CoreSight 10 or CoreSight 20 pinout. Some Cortex-M targets may also provide 20-pin 2.54 mm target debug connector. Be careful with this one since it can have either CoreSight 20 pinout or Arm pinout.