I realize it's been about a month since your post, and I'm sure you've probably already solved the issue, but I figured I'd probably better post this to make sure.

So far, on all TP-Link my TP-Link equipment that has the DoS Protection module, in order for it to work, it says it requires the Traffic Statistics module to also be turned on. If DoS Protection is on, but Traffic Statistics is OFF, the system log will be flooded with this warning. I was fiddling with some settings on my C2600, and forgot to turn the TS module back on. 128 pages of this warning later, and I remember I turned it off, and turn it back on. OOPS!

So, in case you haven't solved it yet, or anyone else is having this issue, check the DoS Protection and Traffic Statistics settings. If DoS Protection is on, Traffic Statistics is probably turned off: just turn it on, and it should fix it. That, or turn off the DoS Protection, your choice.