@duncane:
You might try as well, It might as well be a bug of a policy applied uncorrectly following an upgrade of the firmware.
Changing this settings seemed to have fixed that on my side.
I'll give it a shot tomorrow, I just updated the firmware an hour or so ago, will see if that has changed anything tomorrow after work. If not, I'll enable the guest network and try it, then disable the guest network and try it again.