Heikki Taalikka

I can confirm this in 1.20.2 

I had a wall with item frames and all maps are gone. 

My server is running in local computer by server.jar in version 1.20.2

 
Server output says

[13:10:01] [Server thread/ERROR]: Not a string
[13:10:01] [Server thread/ERROR]: Error loading saved data: map_39
java.lang.IllegalArgumentException: Invalid map dimension: null
at ebw.c(SourceFile:202) ~[server-1.20.2.jar:?]
at java.util.Optional.orElseThrow(Optional.java:403) ~[?:?]
at ebw.b(SourceFile:202) ~[server-1.20.2.jar:?]
at ecc.a(SourceFile:70) ~[server-1.20.2.jar:?]
at ecc.b(SourceFile:57) ~[server-1.20.2.jar:?]
at akq.a(SourceFile:1222) ~[server-1.20.2.jar:?]
at cjm.a(SourceFile:60) ~[server-1.20.2.jar:?]
at akp.a(SourceFile:122) ~[server-1.20.2.jar:?]
at aka.l(SourceFile:1222) ~[server-1.20.2.jar:?]
at ako.t(SourceFile:398) ~[server-1.20.2.jar:?]
at ako.a(SourceFile:326) ~[server-1.20.2.jar:?]
at akq.a(SourceFile:333) ~[server-1.20.2.jar:?]
at net.minecraft.server.MinecraftServer.b(SourceFile:897) ~[server-1.20.2.jar:?]
at ajn.b(SourceFile:283) ~[server-1.20.2.jar:?]
at net.minecraft.server.MinecraftServer.a(SourceFile:820) ~[server-1.20.2.jar:?]
at net.minecraft.server.MinecraftServer.w(SourceFile:667) ~[server-1.20.2.jar:?]
at net.minecraft.server.MinecraftServer.a(SourceFile:263) ~[server-1.20.2.jar:?]
at java.lang.Thread.run(Thread.java:833) ~[?:?]
 

 It cannot load normally saved map data I think. There is many of these logs with difference map name.

I have development version, in running version (.jar) there these class file names are sorted and there not exists .java source files.
I added that comment for Minecraft code peoples because they use these unsorted files.

Can be fix when remove 3 lines from SkinManager.java file.

Remove Lines: 133 - 134, 136

Reason for issue: Condition try check if session id is same as player id so loader cannot been never equal session id than host. So these lines remove then all players gets server owner skin showed.

Tested on my development version.