Can't get out of airplane mode: option is greyed out

When I go to bed, I usually put my Samsung Galaxy Ace in airplane mode to use it as an alarm clock/music player, while saving battery. I can enter that mode successfully.

However, after I enter that mode, sometimes I can't leave that mode since the option (the second one in the list) is greyed out: I have to reboot. Any ideas?

I'm using Android 2.3.4 and my phone is rooted.