Final 6.2.1 update causing LAN instability/sluggishness

So this is my setup.

ReadyNas 314 6.2.2
IP6 disabled
connman 1.26 is installed.

After a short moment the network is flooded with traffic killing router.

Is IPv6 disabled also on the router? Does anything change if you enable IPv6?

There were issues with flooding in 6.2.1, but 6.2.2 had the fix.

Unfortunately, this kind of flood sometimes happens when the NAS has been hacked. So you might need to contact support (even if they charge you).

I swapped out router for an old one. Problem goes. Ip6 definitely off on it, homehub5 is unknown. Could there still be an issue?

You shouldn't be seeing the flood of traffic, and you haven't really identified the cause.

Though it is good that the issue doesn't occur with the old router - seems to me that makes it much less likely that the NAS is compromised.

Do you have a static IP address assigned?

True. It would be good to know the cause. No static IP is set.

Do you know if the flood is on the internet connection or the local LAN side?

A couple of fairly simple tests you could make:
(a) connect the new router behind the old router. It should end up with a different private address space (for instance if the outer router uses 192.168.0.x, the new one might use 10.0.0.x).
(b) reverse the inner and outer routers (putting the new one outside).

Connect the your equipment (at least the NAS) to the inner router.

If (a) floods and (b) doesn't, then that's a pretty clear indication that the NAS is not hacked, and something odd is going on between the NAS and the new router. skywalker might want to know about that, so maybe PM him if that's the outcome.

Also, you might check that any forwarded ports are configured the same way in the two routers.