I've just moved to Zen and had an interesting 24 hours because of this router.
The connection went live around 00:30 and using the connection felt good. Set up the tbb Broadband Quality Monitor and went to bed happy.
Next day I saw this:
Long story cut short: the Fritz box throws a hissy fit if you ping it too quickly and thinks it's some kind of attack.
Set up my NAS box which is always on with IPv6, and am now monitoring that and all looks fine.