Error connecting to snapshot server

My friend (Collisto) was trying to connect to my server but could not. The following is the output from the console.

 

[21:55:00] [INFO] UUID of player Collisto is 0f8eab52-c7d6-4667-9eec-be314b05a8e5

[21:55:00] [WARN] Failed to handle packet for /185.4.25.34:54016

[22:55:00] java.lang.IllegalStateException: Not a JSON Object: null

[22:55:00] at com.google.gson.JsonElement.getAsJsonObject(JsonElement.java:90) ~[minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at wi.a(SourceFile:81) ~[minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at wi.<init>(SourceFile:47) ~[minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at vj.a(SourceFile:923) ~[minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at sz.<init>(SourceFile:160) ~[minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at vj.g(SourceFile:443) ~[minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at ty.b(SourceFile:113) ~[minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at ty.Y_(SourceFile:65) ~[minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at hv.a(SourceFile:218) ~[minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at tv.c(SourceFile:170) [minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at net.minecraft.server.MinecraftServer.w(SourceFile:722) [minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at si.w(SourceFile:356) [minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at net.minecraft.server.MinecraftServer.v(SourceFile:630) [minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at net.minecraft.server.MinecraftServer.run(SourceFile:535) [minecraft-snapshot-1.13-pre4.jar:?]

[22:55:00] at java.lang.Thread.run(Thread.java:748) [?:1.8.0_131]

[21:55:00] [INFO] com.mojang.authlib.GameProfile@79d4dddd[id=0f8eab52-c7d6-4667-9eec-be314b05a8e5,name=Collisto,properties={textures=[com.mojang.authlib.properties.Property@50e1642f]},legacy=false] (/185.4.25.34:54016) lost connection: Internal server error

 

I then contacted the server hosts and after an hour of attempting to fix the issue they rolled the server back to 1.13-pre3, restored the world to a previous backup and told me to report the error on the issue tracker. My friend can now connect but this could cause problems in future updates. Also changing the world did not fix the issue.