Setting RELEASE_MODE breaks the adapter because Usb.Init() is not called.

[sromeroi]

When you uncomment RELEASE_MODE in debug.h to remove all the Serial.print() debugging stuff, the adapter does not work.

The issue is in the setup() function in RFUSB_to_DB15.ino:

void setup() {
  #ifndef RELEASE_MODE
  Serial.begin(115200);

  while (!Serial);

  byte i, j;

  if (Usb.Init() == -1) {
    Serial.print(F("\r\nOSC did not start"));
    while (1);
  }
  #endif                // <----------------------------------------------

  pinMode(LED_PIN, OUTPUT);
}

The #endif statement, placed here, prevents Usb.Init() from being executed, and thus the USB is not initialized.

Instead, it should be:

void setup() {
  #ifndef RELEASE_MODE
  Serial.begin(115200);

  while (!Serial);
  #endif

  byte i, j;

  if (Usb.Init() == -1) {
    Serial.print(F("\r\nOSC did not start"));
    while (1);
  }

  pinMode(LED_PIN, OUTPUT);
}

(You can also wrap the "OSC did not start" error with an #ifdef)

  if (Usb.Init() == -1) {
    #ifndef RELEASE_MODE
        Serial.print(F("\r\nOSC did not start"));
    #endif
    while (1);
  }