Radius server unreachable

[MichaelChen]

vSZ is 5.2.2.0 and total 4,925 APs and the model including R700、R500、R550、R600、R610、R650、T610 and T750. These AP locate in 124 schools.
The vSZ is rebuild for ipv6 and this event keep coming.  Is there solution to solve this event?