This happened to me yesterday on one tablet running Android 6.0 without issue for the prior 2 months on the same network, and my other 4.4.4 devices say nothing is wrong with the network, router or Wifi configuration.
On examining the Xfinity router, it appears Xfinity pushed a new configuration to the router over the past days, as it has new software and the error appears on all Xfinity powered networks on the problem tablet.
The tablet remains surfing without restrictions on Internet despite saying "Connected, No Internet". Its annoying as much as the "!" suggests something is wrong, but the bigger annoyance is Wifi has to re-selected every time the Tablet is turned on again, it can't connect automatically.
I think this both a bug and a legitimate warning of an issue because after I fixed mine, I removed the fix, and the "!" remains OFF.
For example, it works correctly on captive portal wifi such as in McDonalds or Starbucks.
I think the "!" appears with too many possible issues, its very non specific.
There are many solutions.
I tried the typical ones, turning the router off/on, device off/on, resetting wifi off/on etc., nothing helped.
This is finally what worked:
1. Go to "Settings"
2. Go to "Data Usage"
3. Go to drop down menu in the upper right hand corner (:)
4. Go to "Network ...."
5. Toggle ON/OFF what ever setting you see there.
6. Optionally return it to the setting you found to see if its a 'bug'
Its a variation of what was posted here:
The main solution page: