Kicking in the dark here, but this could be due to this particular station having reporting of events enabled and it most likely didn't send the report to the AP for some reason and the AP is reporting that.
Is the performance degradation due to this messages? What kind of performance issues?
Thank you for your answer. We are trying to isolate suspected problem. So we would like to know the meaning of the message in order to relation between the message and the symptom. I see that the msg is not important and no relation with the suspected problem. The symptom we are faced on is client throughput degradation. (less than 300kbps). I know there is a lot of factors so I try to isolate again and submit a support issue later. Thank you for your suggestions.