PIC18 High Priority Interrupt doesn't Interrupt the Low one (xc8)
Altough I'm not sure why, this solved my problem.



At Power-On-Reset, ADON bit is cleared, which means it disables the A/D converter by default, right? This means whether or not I disable the A/D converter, it is disabled automatically.

ADCON0bits.ADON = 0;


But, I did add this line to my configuration and it worked, now both external (high priority) interrupt and USART (low priority) interrupt works at the same program.

It is a solution, but i still wonder why it worked.