I would set the Access Policy logs on debug. If you are on 13.x and above go into Access>Event Logs>Settings, you can change the log level here.
At the same time run a tcpdump on the server to see what is being returned or if APM can not reach the server:
tcpdump -ni 0.0:nnn -s0 ‘((host x.x.x.x) and (port 88 or port 389)) or ((port 88 or port 389 or port 445 or port 464) or (port 53))’ -vvv -w /shared/tmp/ad_test_2019-09-09.pcap