WARNING SAVE SERVER BEFORE CONTINUING
I fixed this problem, I just recently encountered this with my families two oldest minecraft accounts. What I did was look at server log, It said:
[10:43:56 INFO]: UUID of player ************ is aa5aa98e-8e15-4176-9c23-f602d29eb998
[10:43:56 WARN]: Failed to handle packet for /192.168.2.200:63181
i: Non [a-z0-9/._-] character in path of location: minecraft:EntityHorse
at pc.<init>(SourceFile:38) ~[minecraft_server.1.13.jar:?]
at pc.<init>(SourceFile:43) ~[minecraft_server.1.13.jar:?]
at wn.a(SourceFile:121) ~[minecraft_server.1.13.jar:?]
at wn.a(SourceFile:100) ~[minecraft_server.1.13.jar:?]
at wn.<init>(SourceFile:48) ~[minecraft_server.1.13.jar:?]
at vo.a(SourceFile:915) ~[minecraft_server.1.13.jar:?]
at te.<init>(SourceFile:159) ~[minecraft_server.1.13.jar:?]
at vo.g(SourceFile:442) ~[minecraft_server.1.13.jar:?]
at ud.b(SourceFile:113) ~[minecraft_server.1.13.jar:?]
at ud.Y_(SourceFile:65) ~[minecraft_server.1.13.jar:?]
at hw.a(SourceFile:238) ~[minecraft_server.1.13.jar:?]
at ua.c(SourceFile:171) [minecraft_server.1.13.jar:?]
at net.minecraft.server.MinecraftServer.w(SourceFile:749) [minecraft_server.1.13.jar:?]
at sn.w(SourceFile:357) [minecraft_server.1.13.jar:?]
at net.minecraft.server.MinecraftServer.v(SourceFile:656) [minecraft_server.1.13.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:559) [minecraft_server.1.13.jar:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_171]
[10:43:56 INFO]: com.mojang.authlib.GameProfile@739935ca[id=aa5aa98e-8e15-4176-9c23-f602d29eb998,name=********,properties={textures=com.mojang.authlib.properties.Property@7055b05f]},legacy=false] (/192.168.2.200:63181) lost connection: Internal server error
what I did is take the player ID which in this case is aa5aa98e-8e15-4176-9c23-f602d29eb998 as seen in line:
[10:43:56 INFO]: UUID of player ************ is aa5aa98e-8e15-4176-9c23-f602d29eb998
the 10:43:56 is time you can ignore it.
anyway go into your server folder, not the individual folders, but the main folder that holds it all and at the top right you should have windows search. search the folder for the ID: aa5aa98e-8e15-4176-9c23-
f602d29eb998
WARNING SAVE SERVER BEFORE DELETING
when it pops up with all files with that name, delete them all,
WARNING SAVE SERVER BEFORE DELETING
load server and try logging in now
note: all ids are different this one wont work because it is one of mine, try logging into world than look at server log that will give info above. That should say what the name of player is that failed and its ID.

I keep trying to get on a server(not mine) but it keeps saying "internal server error". Other people can get on the server but not me and I've been on the server before. Is there anyway to fix that??

I get exactly the same thing im c-owner talk with the owner weekly and cant get on with "Internal server error" its not my server its the owners and i just cant get on this has been happening for months now ive reported it to mojang and everything please help!

to fix it you need to get your miencraft server on your desktop and put it into ur minecraft saves file. After that mcedit your server spawn or parts into a new minecraft map and upload it to your server. This is happening because its a 1.7.2 bug. If you dont want to do that then just delete your player.dat file. To reach that open up your server file and go into your main server world then go into players and delete your player.dat file! If this doesnt work please tell me.

Guys I signed up JUST to come on this post ISE happens when your serveR is overloaded so either get more RAM or just delete your player files. It worked for me and lots of other people its pretty easy to fix :/

Dear Fellow people, if this happens delete the .Minecraft folder but make sure you take out the folders and items that you need before doing. Now download a new minecraft than let it patch. After that add in your folders and the ips and it should be fixed =) if you still have this problem please reply to this message

I have a server that I run from a hosting service, and I am having the same problem. I keep restarting it, but it still doesn't work.