I know this doesn't sound good, but you could manually set the time on all your APs until you get this resolved. Unfortunately, the time will reset if the APs are rebooted before this is resolved.
You could also try using a different NTP server just to really rule that out as a possible cause.
Also, when you do a "show ntp", does it show the NTP server as "(active)"? And have you wiresharked this to confirm the NTP packets are making their way to and from?
Interval: 1440 minutes
Daylight Saving Time: Yes
Start 03-12 01:59:59
End 11-05 01:59:59
If no NTP server (1st, 2nd, 3rd above) do not have "active" out to the right, the devices are not able to reach the server.