I suspect it might have been caused by the fact that the new access point I installed had v1.16, however all others had v1.00 loaded.
I work in an office of just under 100 users, we have 5 DAP-1665 access points located around the office. We are located in Johannesburg, South Africa in the CBD, so there are many access points in the area.
Each access point is an AP, connecting via a switch to our Fortigate 100D router.
After installing the new access points, a couple of the older ones kept rebooting themselves, these had been working for months prior without any problems. However the new access point was working fine, so I decided initially to upgrade the older ones to v1.16, however then noticed that even the new access point was now rebooting itself. At this point I did a factory restore and loaded v1.00. Have not had any problems on any of the access points since.
Some of the access points have as many as 30 devices connected, across both 2.4GHz & 5GHz channels.
Hope that clarifies the issue we experienced.