Hi fpiSTM,
quite sure, thats why I asked over there on the stm32duino github site
I use HardwareSerial StLinkSerial(PC_11_ALT1, PC_10_ALT1);
nevertheless, it also happens on UART4 on those same pins and on USART1 on PA_10 and PA_9
I will have a look at the errata-sheet of the device, thanks!