As mentioned in #115, tracked down originally over in the EnderCore git issues (link), and in several threads on CurseForge (here is one), TickNextTick list out of synch is a bug that RandomPatches attempts to fix but has some difficulty doing in 1.16.1. This thread may serve as a superthread for its debugging & reporting, as #115 is written to be Enigmatica-specfic. If so desired by the contributors I'll remove this report & put it as a comment into #115.

Exceptions tend to look like the following:

[27Jul2020 03:16:30.570] [Server thread/ERROR] [net.minecraft.s
erver.MinecraftServer/]: Encountered an unexpected exception
net.minecraft.crash.ReportedException: Exception ticking world
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:856) ~[?:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:788) ~[?:?]
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:81) ~[?:?]
at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:643) [?:?]
at net.minecraft.server.MinecraftServer.lambda$func_240784_a_$0(MinecraftServer.java:230) [?:?]
at net.minecraft.server.MinecraftServer$$Lambda$6550/1020321556.run(Unknown Source) [?:?]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_51]
Caused by: java.lang.IllegalStateException: TickNextTick list out of synch
at net.minecraft.world.server.ServerTickList.func_205365_a(SourceFile:54) ~[?:?]
at net.minecraft.world.server.ServerWorld.func_72835_b(ServerWorld.java:329) ~[?:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:852) ~[?:?]
... 6 more

Exceptions are accompanied by a brief freeze (sub-1-second) and a crash-to-launcher.

In my experience, it is more frequent when chunk generation/loading is backlogging server ticks. Examples include fast travel in creative, using /tp commands, and when entering/exiting the Nether in previously ungenerated areas.

Prior to recent commits (faef98 and a5563c) it seemed to me to arise most frequently in the presence of multiple hostile mobs/mob spawners. Since those commits this behavior is absent and crashes are far less frequent (occur on an hourly basis, not minutely basis).

Dunno if this could help ...
https://gist.github.com/OldDogy/8f4d22f4077536672c3a05e32e8abc4a

There were actually 3 of us online flying around the nether. Two of us that didn't run in to the issue got kicked with java stack overflow error. 3rd player crashed with the server.

After your recent fix, it seems to only be occurring in the Nether.

OldDogy, try removing netherportalspread mod, it is modifying things outside the main server thread and can cause out of sync bug.
We removed that mod from our 1.16.1 modpack and haven't seen this error for about a week. With that mod we seemed to crash with this error once a day.
netherportalspread mod has supposedly been fixed

I'm gonna move this to another issue. 😄

I recently experienced this with my mod-pack https://github.com/P3rf3ctXZer0/Pixelmon-Evolved

Latest.log https://gist.github.com/P3rf3ctXZer0/3efd29484e7ab3126e0d35652e51e77f

If helps I am using 1.12.2 Ender IO that is how I found this bug issue.

It happened on a dedicated server not sp.