The Host will try to send info about the baudrate,... via USB to the device. This config needs to be saved somewhere. For some reason, the ST middleware leaves this up to the user (maybe for multiple VCOM on one chip ??). I had the same problems, and found a solution somewhere in the ST Forum. Can't remember where sadly.
Anyway, search for the file "usbd_cdc_if.c". Search for the function "CDC_Control_FS". Replace the following lines into the appropriate cases:
case CDC_SET_LINE_CODING: //a host sends the line encoding to this device, save it
tempbuf[0]=pbuf[0];
tempbuf[1]=pbuf[1];
tempbuf[2]=pbuf[2];
tempbuf[3]=pbuf[3];
tempbuf[4]=pbuf[4];
tempbuf[5]=pbuf[5];
tempbuf[6]=pbuf[6];
break;
case CDC_GET_LINE_CODING: //a host wants to read the saved encoding for this device
pbuf[0]=tempbuf[0];
pbuf[1]=tempbuf[1];
pbuf[2]=tempbuf[2];
pbuf[3]=tempbuf[3];
pbuf[4]=tempbuf[4];
pbuf[5]=tempbuf[5];
pbuf[6]=tempbuf[6];
break;
tempbuf can be static in the function scope or, as I did it, somewhere on global RAM:
/* USER CODE BEGIN PRIVATE_VARIABLES */
uint8_t tempbuf[7];
/* USER CODE END PRIVATE_VARIABLES */
Good luck!
