I have the same issue, R750 and vSZ 5.2.0, and already upgraded to the latest ap patch 5.2.0.0.5010
Enabling debug on R750 I got this and I notice that GET_DOMAIN_SUFIX_INFO resolves to 127.0.0.1, and the ap uses this adress as the "controller" address.
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: util_cmd_proc:1574 Execute CMD type: [1][GET_BASE_INFO].
Mar 16 17:48:52 APR75003 daemon.warn cubic[1487]: apcfg_general_get_base_info:513 ret:[6]/[bad key: device-gps-coordinates] No GPS data
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: apcfg_general_get_sz_list:319 Server list[172.17.128.253,172.30.11.248,] from SZ..
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: apcfg_general_get_sz_list:398 Server list[172.17.128.253,172.30.11.248,,172.30.11.248] appended from lastJoin..
Mar 16 17:48:52 APR75003 daemon.warn cubic[1487]: apcfg_general_get_base_info:565 ret:[34]/[file missing for RSM services] No server cert validation setting.
Mar 16 17:48:52 APR75003 daemon.debug cubic[1487]: apcfg_general_get_radio_info:153 background scan wifi0: en:0, timer:20, channel num:13.
Mar 16 17:48:52 APR75003 daemon.debug cubic[1487]: apcfg_general_get_radio_info:153 background scan wifi1: en:0, timer:20, channel num:20.
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: cub_conn_get_addr_state_action:611 It's Registered.[1].
Mar 16 17:48:52 APR75003 daemon.debug cubic[1487]: cub_conn_get_addr_state_action:615 Current server list:[172.17.128.253,172.30.11.248,,172.30.11.248]..
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: cub_state_action:2315 Start [IMG_REQ_STATE].
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: cub_state_action:2319 [CONN_GET_ADDR_STATE] -> [IMG_REQ_STATE]; reason: [CUB_RESULT_CONN_GET_ADDR_SUCCESS_REGISTERED].
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: json_cmd_proc:558 Data type : [3][GENERATE_IMGREQ].
Mar 16 17:48:52 APR75003 daemon.debug cubic[1487]: curl_https_request:1332 curl_https_request, msg_type:[5][MSG_TYPE_IMGREQ], via ssh:[yes], send_data:[{ "protocolVersion": "0.50", "timeStamp": "20200316T174852", "seqNumber": 109, "zoneUUID": "499b2956-8d7a-4d73-8346-8c886b12dda5", "deviceIpMode": 3, "bkupFwVer": "112.1.0.0.2146", "configUUID": "b427e253-67ac-11ea-ab86-f627fe1aafae", "purpose": "FW_CONF" }].
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: util_cmd_proc:1574 Execute CMD type: [5][GET_DOMAIN_SUFFIX_INFO].
Mar 16 17:48:52 APR75003 daemon.debug cubic[1487]: wrap_getaddrinfo:1187 getaddrinfo with address [127.0.0.1] returns success.
Mar 16 17:48:52 APR75003 daemon.debug cubic[1487]: resolve_addrinfo:1236 wrap_getaddrinfo with addr [127.0.0.1] returns success.
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: do_curl:1069 Send HTTP request to URL:[
http://127.0.0.1:8082/wsg/ap/config/4C:B1:CD:27:84:20/5.2.0.0.5010].
Mar 16 17:48:52 APR75003 daemon.err cubic[1487]: do_curl:1131 curl_easy_perform failed:[7][Couldn't connect to server].
Mar 16 17:48:52 APR75003 daemon.debug cubic[1487]: cub_sm_check_retry_timeout:183 Lost seconds:[236], connection timeout:[300], server num:[2].
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: cub_state_action:2315 Start [CONN_GET_ADDR_STATE].
Mar 16 17:48:52 APR75003 daemon.info cubic[1487]: cub_state_action:2319 [IMG_REQ_STATE] -> [CONN_GET_ADDR_STATE]; reason: [CUB_RESULT_IMG_REQ_CONNECTION_FAIL].
Mar 16 17:48:52 APR75003 daemon.err gapd: Fail to get pool stats
Mar 16 17:48:52 APR75003 daemon.info gapd: selecting.....
Mar 16 17:48:53 APR75003 daemon.err collectd[1495]: Unable to access rsm for retrieving server address