Don't understand wot it...
Version 1.16.1 Spigot

[Craft Scheduler Thread - 132/WARN]: [Autorank] Plugin Autorank v4.4-SNAPSHOT-108 generated an exception while executing task 56 java.lang.NoClassDefFoundError: com/palmergames/bukkit/towny/exceptions/NotRegisteredException at java.lang.Class.getDeclaredConstructors0(Native Method) ~[?:?] at java.lang.Class.privateGetDeclaredConstructors(Class.java:3137) ~[?:?] at java.lang.Class.getConstructor0(Class.java:3342) ~[?:?] at java.lang.Class.newInstance(Class.java:556) ~[?:?] at me.staartvin.utils.pluginlibrary.Library.getHook(Library.java:110) ~[?:?] at me.staartvin.utils.pluginlibrary.PluginLibrary.getLibrary(PluginLibrary.java:43) ~[?:?] at me.armar.plugins.autorank.hooks.DependencyManager.getLibraryHook(DependencyManager.java:181) ~[?:?] at me.armar.plugins.autorank.hooks.DependencyManager.isAFK(DependencyManager.java:95) ~[?:?] at me.armar.plugins.autorank.playtimes.UpdateTimePlayedTask.run(UpdateTimePlayedTask.java:55) ~[?:?] at org.bukkit.craftbukkit.v1_16_R1.scheduler.CraftTask.run(CraftTask.java:81) ~[spigot-1.16.1.jar:git-Spigot-05da6fa-b0c6dfe] at org.bukkit.craftbukkit.v1_16_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:54) [spigot-1.16.1.jar:git-Spigot-05da6fa-b0c6dfe] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?] at java.lang.Thread.run(Thread.java:834) [?:?] Caused by: java.lang.ClassNotFoundException: com.palmergames.bukkit.towny.exceptions.NotRegisteredException at java.net.URLClassLoader.findClass(URLClassLoader.java:471) ~[?:?] at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:167) ~[spigot-1.16.1.jar:git-Spigot-05da6fa-b0c6dfe] at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:96) ~[spigot-1.16.1.jar:git-Spigot-05da6fa-b0c6dfe] at java.lang.ClassLoader.loadClass(ClassLoader.java:589) ~[?:?] at java.lang.ClassLoader.loadClass(ClassLoader.java:522) ~[?:?] ... 14 more

When does this error occur? Do you have Towny installed or are you using Towny requirements?

The pattern of occurrence of the error is not found. Towny do not use. But there is a Reputation plugin. He also introduces a hierarchy. Maybe because of him. I'll try to disable it today.

When the server rebooted, Autorang issued a conflict with the PermissionsEX plugin, which is not supported. But at 1.15, everything worked perfectly. Apparently the whole problem was exactly that. I will look for another permission plugin.
Thanks

After rearranging plugins and reconfiguring, the error did not recur. I still do not understand what has towny to do with it, if it was not there. I think the problem is solved.