Slow Wifi and error message : Failed to stop TX DMA, queues=0x002!

Hi,

I have problem with Wifi (8  AP55 / 5 SSID 2.4Ghz and 5 GHz / UTM9), in metting room sometimes wireless clients (not all, i think 2.4 Ghz clients) have problems slow internet and disconnect connection.

I'm now in test and i reveal the problem is recurrent with AP (ID A400487B39DB1FB).

Part of logs :

Any suggestions ? Possible AP is physical down ?

PS : Sorry for my poor English...