Enable TLS debugging in the server to troubleshoot SSL communication issues:

$ dsconfig create-debug-target \
  --publisher-name "File-Based Debug Logger" \
  --target-name com.unboundid.directory.server.extensions.TLSConnectionSecurityProvider \
  --set debug-level:verbose \
  --set include-throwable-cause:true
$ dsconfig set-log-publisher-prop \
  --publisher-name "File-Based Debug Logger" \
  --set enabled:true \
  --set default-debug-level:disabled

In the java.properties file, add -Djavax.net.debug=ssl to the start-server line, and run bin/dsjavaproperties to make the option take effect on a scheduled server restart.