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I have tested it again.

The reason is, that I am from germany and have a german keyboard. To type "\" I must hold down "Alt Gr" and one of the number keys.

The problem ist, that after I have typed the [alt] key, compile.exe not recognized, when it is no longer pressed.
So, one time typed the [alt] key, every time [enter] is pressed, is like and [alt]+[enter].