Can you please post your complete project or a shortened version that shows the error so that we all can have a look at all of your settings. To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file.
An issue may be that you do not wait for a byte ready. Try using the UART_GetRxBufferSize() API.
while(UART_GetRxBufferSize() < size_start_num)
and fix it. but looks like the speed to get out this while loop is very slow
Your loop ends when all characters are transmitted. Since UART transmission al lower baud rates are comparably slow when looking at the processor clock (several MHz). At 9600 baud the transmission of a single byte will take 1.25 ms.