May this helps:DescriptionSymptom:Two symptoms -- 9115 AP stuck in reboot loop with tracebacks.- AP doesn't get CDP of Switch (even though Switch gets the AP's CDP). Error "MDIO Error: MDIO got failure status on phy 6" occasionally appears on console...
I had the same problem.I enabled fastSSID change there was not any problem like AP reboot and client disconnect.I had Flex APs too.There is no warning in config guide :https://www.cisco.com/c/en/us/td/docs/wireless/controller/7-4/configuration/guides...