I've just installed the current release of RockBox (3.9.1) on my Ipod Nano 1st gen and the problem appeared. It's the same as in IpodFAQ:
My 1st or 2nd generation iPod doesn't shut down, it only reboots. What's up with that?
The 1st and 2nd gen iPods don't have proper shutdown support in Rockbox.
But I have Nano, not these versions in FAQ.
When player is turned on, I press and hold the play button, Shutting down message apears, and then the player suddenly blink (like when the battery is very low and it shuts down by its own) and Apple logo appears instantly, as if I were reloading the player by holding Select+Menu buttons.
I tried to install different older releases, it doesn't help.