Did you upgrade to v5.2 just your FortiGate box or both - FG and FAP?
The thing is that FortiOS 5.2.0 release notes explicitly says that " The FortiAP device must be running FortiAP v5.2.0 build 0212 or later." On the other hand, FortiAP v.5.2.0 release notes says that " FortiAP v5.2.0 is supported by FortiOS v5.2.0" but doesn' t say anything about FortiOS v.5.0.X support. The way we usually implement upgrades of Fortinet devices throughout our network environment is - first we upgrade all our FortiAP units. And after a while (it could be months), if everything works fine, we go ahead and upgrade FortiGate and FortiWiFi units FortiAPs are connected to. The question is whether FortiAP v5.2.0 is supported by FortiOS v.5.0?
Anyway, did you manage to resolve your FAP-221C issues?