Full report and potential offending code.
https://gist.github.com/P3rf3ctXZer0/15f9c3ade499b0c1dd0053515b0bc72f

[08Jun2020 22:53:11.827] [Server thread/ERROR] [net.minecraft.server.MinecraftServer/]: Encountered an unexpected exception
net.minecraft.crash.ReportedException: Ticking block entity
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:853) ~[?:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:330) ~[?:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:784) ~[?:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:637) [?:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_241]
Caused by: java.lang.NullPointerException
at net.morimori.imp.file.PlayList.getWorldPlaylistNBTDataString(PlayList.java:181) ~[?:1.1]
at net.morimori.imp.tileentity.CassetteDeckTileEntity.finishRecoding(CassetteDeckTileEntity.java:80) ~[?:1.1]
at net.morimori.imp.tileentity.CassetteDeckTileEntity.func_73660_a(CassetteDeckTileEntity.java:170) ~[?:1.1]
at net.minecraft.world.World.func_217391_K(World.java:1407) ~[?:?]
at net.minecraft.world.server.ServerWorld.func_72835_b(ServerWorld.java:368) ~[?:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:849) ~[?:?]
... 4 more

Would you please send the music file with the error?

Would you send a NBT file at world/iammusicplayer/playlist.dat?

How would I do that?

Music file (worked fine in single player)
https://www.dropbox.com/s/ntl7fvovfgac2ly/%23NL%20-%20Sonic%20Unleashed%20-%20Apotos%20%5BWindmill%20Isle%20Day%5D.mp3?dl=0

Even more raw version (Had to zip it though github policy)
#NL - Sonic Unleashed - Apotos [Windmill Isle Day].zip

Not sure if this helps but I just copied the whole "iammusicplayer" folder and its contents to server after using it in single player.

Strange playlist.dat does not exist. Also the folder labeled picture is mislabled as "pictuer"
iammusicplayer.zip

This is the zip from the server.

Could not pinpoint the cause.
Please delete "playlist.dat" and execute "/worldplaylistreload".
If that doesn't work please let us know.

playlist.zip

I think its a ticking entity bug. NBT looks fine.

Another nbt image from a different nbt reader

I already did that twice.

https://www.youtube.com/watch?v=q94DLFbW8Dg

No Error
https://youtu.be/wAxO7T9ZcLk

If you don't mind, can you tell me the OS and specifications that the Minecraft server is running?

We have a test environment only on Windows, so we can not handle it if the error is on another OS.

Windows 10

The video shows the crash and how I got there even shows my server folder. Do you have a discord so we can solve this more hands on? I have helped many modders with obscure bugs in the past been around since 1.5.2 and I only recently jumped from 1.12.2 to 1.15.2 and I am noticing things are breaking in ways I am not familiar with. I am familiar with cause and effect though. I have the time to test if you do too.

I am trying to find you some help from very skilled contacts. I will let you know what I discover.

https://www.minecraftforge.net/forum/forum/70-modder-support/

I understood the cause, so I will fix it in the next version

https://www.curseforge.com/minecraft/mc-mods/iammusicplayer/files/2982273
Released a fixed new version.

My issue is on windows not mac or linux will your fix still work?

maybe OK.
If not, please report it

Alright leave this open I will be testing shortly.

Seems to work fine now :D