7025 Random Discconect from Server

unknown

I have 2 Controller ZD 3050 with version 9.8.3.0 build 14 (I know this is old) 
Since Yesterday, our AP 7025 is randomly Disconnect (i cannot get the pattern) 

How is it possible although we not change setting on our ZD. 

please help 

unknown

it might be a switch issue. What kind of switches are they connected to?
Also, make sure the APs can connect over FTP to the ZD