If found a solution for my problem, but don't really understand why it works!!

On initialisation I set the interrupt enable bit of the ADC and RTC to 1.

Until the first interrupt of the RTC (after 1 second) arises, the ISR of the faster ADC unit are serviced.

Now after the first RTC interrupt,the ADC interrupts stop.

Now i set the interrupt enable bit of the RTC back to 0 (in the ISR of RTC) then again the ISR of the ADC gonna be serviced. In the ISR of the ADC I set the interrupt enable bit of the RTC back to 1.

Both Interrupts are serviced fine, but why??
The interrupt flags of RTC and ADC are both set back to 0 by software before too (too before i did the changes with the interrupt enable bit)

What has the interrupt enable bit to do with this??