Finally I can confirm that AP patch version 220.127.116.11.1038 fixed "Radius unreachable" message problem for me.
I installed it 2 weeks ago, and messages "Radius server 127.0.0.1 unreachable" are not coming any more. There were previously similar messages with different IPs, and they are also not present any more.
Patch was available through support before, but for a few days it is available on support site for download. To achieve result, you need to apply new firmware to each zone (as it is actually fixed AP firmware, if APs are on older version, patch will have no effect).
You need 5.2.1 version to use a patch. I am not sure if similar patch is available for older releases.
In the end of December 5.2.2 is expected, which will include this patch.