We had a player enter the end on our server after updating to the latest snapshot. He immediately got disconnected and when he reconnected he got an "Internal server error" message every time.

Log output

[01:01:04] [Server thread/INFO]: <player>[/<ip>] logged in with entity id <hidden> at (100.0, 49.0, 0.0)
[01:01:04] [Server thread/INFO]: <player> joined the game
[01:01:05] [Server thread/INFO]: [STDOUT]: Negative index in crash report handler (0/9)
[01:01:05] [Server thread/WARN]: Failed to handle packet for /<ip>
f: Ticking player
at nu.k_(SourceFile:398) ~[minecraft_server.17w17ajar:?]
at oe.G_(SourceFile:182) ~[minecraft_server.17w17ajar:?]
at gf.a(SourceFile:231) ~[minecraft_server.17w17ajar:?]
at od.c(SourceFile:187) [minecraft_server.17w17ajar:?]
at net.minecraft.server.MinecraftServer.D(SourceFile:652) [minecraft_server.17w17ajar:?]
at nd.D(SourceFile:335) [minecraft_server.17w17ajar:?]
at net.minecraft.server.MinecraftServer.C(SourceFile:562) [minecraft_server.17w17ajar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:466) [minecraft_server.17w17ajar:?]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_121]
Caused by: java.lang.NullPointerException
[01:01:05] [Server thread/INFO]: jkh lost connection: TextComponent{text='Internal server error', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, un
derlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[01:01:05] [Server thread/INFO]: <player> left the game

Stopping the server, downgrading to 17w16b fixed the issue. No world data was lost or corrupted.