Specific players on server go into a "spectator" mode

My friend, after dying sometimes, will be able to move around and play for about 5 seconds. After those 5 seconds, he goes into a sort of "Spectator mode" that looks like survival. He can't do any thing or move. He can only punch. He and his body are in different places. If he dies or leaves and tries to join again, he just times out.

This is the error code we got:

[Server thread/FATAL]: Error executing task on Server
java.lang.NullPointerException: null
at bns.l(SourceFile:173) ~[server.jar:?]
at bns.d_(SourceFile:421) ~[server.jar:?]
at aav.a(SourceFile:495) ~[server.jar:?]
at zc.a(SourceFile:1216) ~[server.jar:?]
at qv.a(SourceFile:30) ~[server.jar:?]
at qv.a(SourceFile:8) ~[server.jar:?]
at na.a(SourceFile:21) ~[server.jar:?]
at ug.run(SourceFile:18) ~[server.jar:?]
at akx.c(SourceFile:144) [server.jar:?]
at alb.c(SourceFile:23) [server.jar:?]
at net.minecraft.server.MinecraftServer.b(SourceFile:749) [server.jar:?]
at net.minecraft.server.MinecraftServer.c(SourceFile:142) [server.jar:?]
at akx.w(SourceFile:118) [server.jar:?]
at net.minecraft.server.MinecraftServer.bc(SourceFile:731) [server.jar:?]
at net.minecraft.server.MinecraftServer.w(SourceFile:725) [server.jar:?]
at akx.c(SourceFile:127) [server.jar:?]
at net.minecraft.server.MinecraftServer.v(SourceFile:710) [server.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:662) [server.jar:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_221]