XMC4400-F64K512 AB use pin RTC_XTAL1 input to generate interrupt using ERU0.1B1

I would like to generate an interrupt on ERU0_0_IRQn on falling edge of an input signal on pin RTC_XTAL1. The Port I/O function table says RTC_XTAL1 is linked to ERU0.1B1. However, it does not seem to work. I am confident that my ERU is configured correctly, as I can make it work with any GPIO pins. So, the problem seems to come from the pin RTC_XTAL1.

XTALGPI1SEL is set to 0 in SCU.HDCR. Is there any other register I should look at to make the RTC_XTAL1 pin work with ERU?

Thanks,
Cedric