While LEDs on GPIO2 work OK, but on the GPIO1 with LPC1768

I have a custom board with device LPC1768. While every other functionalities (such as LCD on SPI, Ethernet, I2C EEPROM, etc.) are OK, for some reason, while I can turn ON and OFF LEDs on the GPIO2 but I can't do the same for the pins on the GPIO1. Maybe I shouldn't blame whole GPIO1 (or myself) but at least the part I try to use.

If I connect a button on one of the the GPIO1 pins (with internal pull-ups and as an OUTPUT) I can read the button inputs as it should be.

Here is the definition part as per the user manual;

#define FIO_BASE_ADDR       (0x2009C000)
#define FIO1CLR             (*(volatile unsigned int  *)(FIO_BASE_ADDR + 0x3C))
#define FIO2CLR             (*(volatile unsigned int  *)(FIO_BASE_ADDR + 0x5C))
#define FIO1SET             (*(volatile unsigned int  *)(FIO_BASE_ADDR + 0x38))
#define FIO2SET             (*(volatile unsigned int  *)(FIO_BASE_ADDR + 0x58))

While a LED on the pin P2.5 turns ON and OFF as it should be;

FIO2SET = (1uL << 5); // P2.5
FIO2CLR = (1uL << 5); // P2.5

A LED on P1.18 doesn't turn ON;

FIO1SET = (1uL << 18); // P1.18

In order to test all the LEDs connected to the GPIO1 pins, I wrote two lines of code using LPC17xx.h and then proofed that it works, all the LEDs are turning ON and OFF accordingly.

LPC_GPIO1->FIOSET = 0xffffffff;
LPC_GPIO1->FIOCLR = 0xffffffff;

Any input highly appreciated.