When I create a server or a LAN world, nobody can join. Their client crashes as soon as it loads.

Here is the log:

14:54:04 launcher Preparing to launch minecraft client for 18w07c
14:54:04 launcher Checking installations.
14:54:04 launcher Minecraft client 18w07c is ready to start.
14:54:04 launcher Starting!
14:54:05 launcher Using default game log configuration client-1.12.xml (outputs XML)
14:54:06 bxe Setting user: Tim_Buckrue
14:54:08 bxh Skipping bad option: lastServer:
14:54:08 bxe LWJGL Version: 3.1.2 build 29
14:54:09 tz Reloading ResourceManager: Default
14:54:10 cwj Missing sound for event: minecraft:block.pumpkin.carve
14:54:10 cwj Starting up SoundSystem...
14:54:10 cwj Initializing No Sound
14:54:10 cwj (Silent Mode)
14:54:10 cwj OpenAL initialized.
14:54:10 cwj Sound engine started
14:54:13 cti Created: 1024x512 textures-atlas
14:54:15 com.mojang.text2speech.NarratorWindows Narrator library for x64 successfully loaded
14:54:15 net.minecraft.realms.RealmsBridge Realms module missing
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 ze Unable to resolve BlockEntity for ItemStack: chiselsandbits:chiseled_cloth
14:54:17 wt Something went wrong upgrading!
i: Non [a-z0-9/._-] character in path of location: modname:PlutoniteIngot
at nx.<init>(SourceFile:36)
at nx.<init>(SourceFile:41)
at yg.a(SourceFile:17)
at wt.b(SourceFile:55)
at wt.a(SourceFile:40)
at wu.b(SourceFile:319)
at akc$1.a(SourceFile:797)
at wt.c(SourceFile:66)
at wt.a(SourceFile:41)
at btp$1.a(SourceFile:87)
at wt.c(SourceFile:66)
at wt.a(SourceFile:41)
at wt.a(SourceFile:33)
at bto.a(SourceFile:94)
at bto.c(SourceFile:76)
at btl.b(SourceFile:64)
at cdt.g(SourceFile:35)
at cdt.<init>(SourceFile:27)
at cdr.c(SourceFile:48)
at cau.a(SourceFile:310)
at bxe.a(SourceFile:728)
at caz$4.a(SourceFile:223)
at byf.mouseClicked(SourceFile:102)
at bzb.mouseClicked(SourceFile:44)
at caz.mouseClicked(SourceFile:507)
at bxg.b(SourceFile:65)
at bxg$$Lambda$369/1819912129.run(Unknown Source)
at cau.a(SourceFile:428)
at bxg.a(SourceFile:65)
at bxg$$Lambda$292/995760621.invoke(Unknown Source)
at org.lwjgl.glfw.GLFWMouseButtonCallbackI.callback(GLFWMouseButtonCallbackI.java:23)
at org.lwjgl.system.JNI.invokeV(Native Method)
at org.lwjgl.glfw.GLFW.glfwWaitEventsTimeout(GLFW.java:2599)
at bxo.a(SourceFile:294)
at bxe.b(SourceFile:845)
at bxe.a(SourceFile:381)
at net.minecraft.client.main.Main.main(SourceFile:141)
14:54:36 ccr Invalid icon for server SkittleMC (SkittleMC.com)
java.lang.IllegalArgumentException: Illegal base64 character a
at java.util.Base64$Decoder.decode0(Base64.java:714)
at java.util.Base64$Decoder.decode(Base64.java:623)
at ccr.g(SourceFile:214)
at ccr.a(SourceFile:146)
at byr.a(SourceFile:33)
at byw.a(SourceFile:391)
at byw.a(SourceFile:194)
at cco.a(SourceFile:310)
at ckd.a(SourceFile:987)
at bxe.b(SourceFile:820)
at bxe.a(SourceFile:381)
at net.minecraft.client.main.Main.main(SourceFile:141)
14:54:37 cad Connecting to localhost, 25565
14:54:40 bxe Unreported exception thrown!
java.lang.NullPointerException
at vn$a.a(SourceFile:77)
at vn$a.a(SourceFile:63)
at axq.a(SourceFile:210)
at bfs.a(SourceFile:263)
at abd.a(SourceFile:1105)
at ckd.b(SourceFile:1183)
at ckd.a(SourceFile:1083)
at ckd.a(SourceFile:944)
at bxe.b(SourceFile:820)
at bxe.a(SourceFile:381)
at net.minecraft.client.main.Main.main(SourceFile:141)