THANK YOU for your guidance.
Server console showed long list of errors; server stopped, had difficulty restarting server.
Not sure what caused it.
Recent Changes: I recently created 2 new ranks with PermissionsEx, and gave 2 new player promotions.
The error seems to happen only when 1 of my recently promoted players is on. I do not see suspicious activity from the player.
*No crash report

I do not know how to obtain thread dump file.

I attached the most recent log, during the most recent error occurrence.

[Spigot Watchdog Thread/ERROR]: The server has stopped responding![Spigot Watchdog Thread/ERROR]: The server has stopped responding![14:17:27] [Spigot Watchdog Thread/ERROR]: Please report this to http://www.spigotmc.org/[14:17:40] [Spigot Watchdog Thread/ERROR]: Be sure to include ALL relevant console errors and Minecraft crash reports[14:17:50] [Spigot Watchdog Thread/ERROR]: Spigot version: git-Spigot-a1ba5fe-b4230a9 (MC: 1.13.2)[14:17:58] [Spigot Watchdog Thread/ERROR]: ------------------------------[14:18:04] [Spigot Watchdog Thread/ERROR]: Server thread dump (Look for plugins here before reporting to Spigot!):[14:18:35] [Spigot Watchdog Thread/ERROR]: ------------------------------[14:18:39] [Spigot Watchdog Thread/ERROR]: Current Thread: Server thread[14:18:53] [Spigot Watchdog Thread/ERROR]: PID: 27 | Suspended: false | Native: false | State: RUNNABLE[14:18:58] [Spigot Watchdog Thread/ERROR]: Stack:[14:19:09] [Spigot Watchdog Thread/ERROR]: java.util.HashSet.<init>(HashSet.java:106)[14:19:14] [Spigot Watchdog Thread/ERROR]: com.google.common.collect.Sets.newHashSet(Sets.java:215)[14:19:22] [Spigot Watchdog Thread/ERROR]: net.minecraft.server.v1_13_R2.AttributeModifiable.<init>(SourceFile:29)[14:19:28] [Spigot Watchdog Thread/ERROR]: net.minecraft.server.v1_13_R2.AttributeMapServer.c(SourceFile:41)[14:19:35] [Spigot Watchdog Thread/ERROR]: net.minecraft.server.v1_13_R2.AttributeMapBase.b(SourceFile:31)[14:19:37] [Spigot Watchdog Thread/ERROR]: net.minecraft.server.v1_13_R2.AttributeMapServer.b(SourceFile:30)[14:19:42] [Spigot Watchdog Thread/ERROR]: net.minecraft.server.v1_13_R2.EntityInsentient.initAttributes(EntityInsentient.java:81)[14:20:05] [Spigot Metrics Thread/WARN]: Exception in thread "Spigot Metrics Thread" [14:20:13] [Spigot Watchdog Thread/WARN]: Exception in thread "Spigot Watchdog Thread" [14:20:19] [Spigot Metrics Thread/WARN]: java.lang.OutOfMemoryError: Java heap space[14:20:26] [Spigot Metrics Thread/WARN]: at java.net.InetSocketAddress.<init>(InetSocketAddress.java:224)[14:20:29] [Spigot Metrics Thread/WARN]: at sun.net.NetworkClient.doConnect(NetworkClient.java:180)[14:20:34] [Spigot Metrics Thread/WARN]: at sun.net.www.http.HttpClient.openServer(HttpClient.java:463)[14:20:41] [Spigot Metrics Thread/WARN]: at sun.net.www.http.HttpClient.openServer(HttpClient.java:558)