Client sometimes is disconnected from server

[bzzbzzb]

[09:40:32] [INFO ] Connection close. SlotType 1, UserNum XXX, CloseType 21, ErrorType 121
[09:40:36] [INFO ] Connection close. SlotType 1, UserNum XXX, CloseType 21, ErrorType 121

I am facing this issue on my server, I found that it might relate to IO Operation of Network communation (guessing )
However I am not really experienced and expert on Network communation and infra stuff.
Perhaps, you guys have faced the same issue and have ideas how to fix it.

Thanks in advance

[MentaL]