PIC18F66K22 - RTCC with external crystal
My RTCC is now running!
By some proper debugging I found out that RTCWREN wasn't being set, thus I could never set RTCEN.
It seems that the unlocking sequence for the RTC write had to have an interrupt disable.
The assembly code from the datasheet wasn't directly useable in my code nor was this described in the datasheet, but some trial and error solved that aswell.
I did not have to set LAT/input/output for the osc pins.

But mind you should set the right configuration bits (#pragma config RTCOSC = SOSCREF // RTCC Clock Select (RTCC uses SOSC)

void rtccEnable(){
    rtccUnlock();//RTCWREN can only be written with the unlock sequence (see Example 18-1).

    RTCCFGbits.RTCEN = 1;

    rtccLock();
}

void rtccUnlock(){//RTCWREN can only be written with the unlock sequence (see Example 18-1).
//Refer to Section 18.0 “Real-Time Clock and Calendar (RTCC)” for the unlock sequence (Example 18-1).
    _asm
    BCF INTCON, 7,1
    MOVLW 0x55
    MOVWF EECON2,1
    MOVLW 0xAA
    MOVWF EECON2,1
    bsf RTCCFG,5,1
    _endasm
}
void rtccLock(){
    RTCCFGbits.RTCWREN = 0;
    INTCONbits.GIE = 1; //Re-enable interrupts.
}

void rtccDebug(){
    xportSendTextDebug("-----[RTCC]-----");
    if(RTCCFGbits.RTCEN){
        while((BOOL)RTCCFGbits.RTCSYNC){}
        xportSendTextDebug("[RTCC] is on");
    }else{
        xportSendTextDebug("[RTCC] is off!");
    }
}