Leonardo in USB slow Mode

I do not know the solution for my problem. I need to emulate a USB keyboard. So far, nothing complex. BUT: It is supposed to be a USB 1.1 keyboard! I know that

UDCON = (1 << LSM)

should be used. But that alone does not make the trick. The USB Device (By the way: it is an Arduino Leonardo) is not found, anyway.

Can anyone please help me?

UDCON = (1 << LSM)

should be used.

Should it? Where?

Post the code of what you have written and we could try and see where you are going wrong.
Read how to use this forum to find out how to do this correctly.

What is USB slow mode, that is not one of the official modes of the USB specification.

Should it? Where?

ATmega16U4 / ATmega32U4 data sheet. Page 260

21.8 Speed Control The speed selection (Full Speed or Low Speed) depends on the D+/D- pull-up. The LSM bit in UDCON register allows to select an internal pull up on D- (Low Speed mode) or D+ (Full Speed mode) data lines.

My Code? It does not matter! Look here:

void setup () {
   Keyboard.begin ();
}
void loop () {
    delay (1000);
    Keyboard.print ("X");
}

The "magic" is to switch to the Leonardo from USB 2.0 to 1.1.

My Code? It does not matter!

You won't get much help here with that attitude. Provide the necessary information or help yourself.

The "magic" is to switch to the Leonardo from USB 2.0 to 1.1.

Explain why this is necessary. The keyboard class usually works, if it doesn't for you, tell us what you're connecting to and why it doesn't work in default mode. Maybe you're wrong that the problem is in the USB speed mode?