Variable arguments via STM32 via USB
That looks correct. Notice the \0 in the string:

"Auto toggle 2 134229731 !\0§ª\232KÖ-çPä(\215õ\221º=\23…"
                          ^^


The \0 is a terminator. The debugger is displaying the rest of the (uninitialized) content in the buffer, but it's not part of the C string that will be printed.

Whatever issue you're having is being caused by something else.