R600 stops associating users via 2.4GHz

[wireless_admin]

Approximately once a day one of our R600 APs connected to ZD 1205 stops associating clients via 2.4 medium. 5GHz clients continue to work with no issues. All other APs work fine.
Issue starts with 2.4 clients loose about 40-60% traffic on their interfaces (Windows, Linux etc.). When you try to reconnect, association doesn't pass.
No RF pollution was noticed on 2.4GHz channel