Is it possible to enable both UART and PLL at the same time on PIC18F?
As already pointed out by the comments, you have to adjust your baud rate settings for the UART to account for the change in frequency by a factor of 4. I usually use something like

// baudrate = ~115,2k
if (PLLEN) {
    SPBRGH1 = 0x00;
    SPBRG1 = 0x89;
} else {
    SPBRGH1 = 0x00;
    SPBRG1 = 0x22;
}


in my UART initialization to have it always work (of course you have to configure PLLEN first).