STM32 VCP - Receive data from PC
I found the answer for my own question. There is a well documentated example on the following webpage:
http://visualgdb.com/tutorials/arm/stm32/usb/

First of all, I had to complete the generated code in static int8_t CDC_Receive_FS (char* Buf, uint32_t *Len) as it is written in the source above.

After that I had to create and implement the int VCP_read(void *pBuffer, int size) function, as it is written in the above mentioned source.

The key point was, that I had to call periodically the int VCP_read(void *pBuffer, int size) function, to read out from the buffer the received data. If I don't call this function, the PC can't write next time the COM port.