Hi,
i have seen that you have Long Guard Interval... Actually in my network that i have with 7000 clients I dont have 802.11b clients... yo may only configure Radio 2 with g/n.
By the other side, actually is posible that you have probles with probe suppresion.
https://gtacknowledge.extremenetworks.com/articles/Solution/Access-points-getting-into-a-locked-stat...
I dont see 9.21.08 version... Is it published now?
In avanced settings of Radio 2 you have STBC, LDBC, ABBDDA support that the help says that is recommended to be enabled. Consider see the help in each window to enable or disable your specific options.
I have more options configured different than yours, but depends of each network.
Apologize for my english