I have a similar issue with 5Ghz (haven't tested if the issue happens with 2.4 yet).

Devices which still connect may take longer to connect (like an ipad 2), and another device gets disconnected more often than not, BQ M10 tablet, while mobile phones nexus 5 and xiaomi redmi note 3 pro seem unaffected.

Reboot seems to fix the issue for devices with issues, but after around 2 weeks the thing persists. I will leave device without rebooting to see if it's general degradation over time or what.

Current firmware version: 1.0.0 Build 20151124 Rel. 61417
Hardware Version: AP500 v1.0

-Confirmed, last night, even though some devices stayed connected, speed tests through wifi gave a mere 5Mbps download 1Mbps upload, after rebooting the AP500, speeds increased to the maximun I can achieve 40Mbps download, 4 Upload.There is a serious bug that causes degradation to this device.

Can eValid measure two or more mobile devices at the same time?

In my case after some tweaking I noticed what was causing this behaviour was changing the default beacon interval to 300 (from 100), I restored it to 100 and the problem hasn't happened again, so... :-?

Try setting Group Key Update Period on every SSID to 0.