@PoP: Is it systematic (works for one book after a reboot, then nada), or does it happen randomly, after a bit of time, or a precise event?
There's quite a bit of papering over weird stuff in the K2/3/4 version, so it's not entirely out of the realm of possibility
. (That, and I mostly field tested this on a K4).
Do you have USBnetwork installed (more specifically, are you willing to check out what state the hack is in over SSH when it breaks)?
In the meantime, you can try enabling the verbose mode (drop a blank 'verbose' file in the linkss folder, then reboot), see if anything pops out on the status print on the bottom of the screen.