Depending on AP model heartbeat loss may be a firmware issue (a lot of those issues have been fixed in firmware) or a more fundamental network issue. I'd start by getting to the latest firmware and go from there. There is also an advisory available suggesting you disable green ethernet options on the switch if you see this. This isn't the advisory I was thinking of but recommends the same solution
https://support.ruckuswireless.com/answers/000006303Are all these WLAN's are on the same VLAN? If not you'll need to setup IP Helpers to point to whatever your DHCP server is on the gateway for that subnet/vlan
That's likely either that one of the two devices has an incorrect timezone set (likely the SZ100) or possibly your daylight savings rules have changed and you need a newer firmware that includes those updated rules
What country code is set for the zone they are joining? My guess is your H500's are country code locked and that the country code of the zone they are being put in is set to something other than they are locked too