on the other serial ports >=Serial1 every single baud rate of the above mentioned >=230400 is expected to work, just with Serial.begin(x) nothing of them?
Um. I don't think that the Arduino core code supports the fractional baud rate divisors needed for this on the USARTs; it only supports the same mechanisms used by the (dumber) UART. The HARDWARE allows it...
If so, then that's a pity, because I use Serial() via the USB port to the 2nd device.
If you're going via the USB/Serial port, then you have additional limitations imposed by the USB/Serial chip (which is an AVR, on Due.)