AP join failed reason by conflict

dear all,

We deployed AC and AP in separate location, lets say AC in location A, and the AP in location B.
So both AC and AP is behind a NAT Router.
We acomplished so AP can join to AC and the capwap tunnel works normally, here is the status of capwap tunnel :
XXXXXX#show capwap detail
CAPWAP process "capwap 1" with state [ Run ]
Process uptime is 0 days 0 hours 56 minutes 4 seconds


We set the AP to goes to group-ap lets say GROUP-A. It's works perfect.

But suddeny when AP goes down ( let say electrical failure ) and then comes up the AP goes to DEFAULT ap group, not in GROUP-A.
in the AC we saw this :
Ruijie-AC#show ap-config summary deny-ap
Deny ap num: 22
Mac Address    AP Name                                  Reason            Timestamp
-------------- ---------------------------------------- ----------------- --------------------
XXXX.XXXX.337b XXXX.XXXX.337bsaridoorlurejo.cef0        By conflict       2020-03-19 16:49:14



I had google about it and I found that :
By conflict             //Indicates that the AP name conflicts with the MAC name.It is because the AP name has already existed on the AC or other APs of thisMAC are online or configured.

I still dont know what thats the meaning, can some body elaborate more about that.

best regard, thank you very much.
sorry for bad english

Hi sir,

Please try to remove the previous AP information on GROUP-A first,
After it be online, move it  back to group-A

best regards
GTAC

Hi sir,
Thank you for your reply.

On my team check, we found that the AP going in to Default group, and there are no AP (neither status is offline or online AP) in the GROUP-A.

And yes we move back the AP from Default group to GROUP-A again, and its running up.

we still dont know the reason and how to prevent this event again.
we hope it doesnt happen again.