Yesterday, me and a friend were playing in the server just fine. I left for 10 or 15 minutes, as well as my friend. When I came back and tried to log into the server, it gave me the error 'Internal server error'. I've restarted the server several times, as well as changing it from vanilla to Spigot. The server is in 1.13 and is plain vanilla. And the error happens to me and to my friend. We've googled a lot but we found no solution or similar problem. Here's the log part where I think the error is:
[19:35:57] [Server thread/WARN]: Failed to handle packet for /37.189.59.244:53446
java.lang.IllegalStateException: Not a JSON Object: null
at com.google.gson.JsonElement.getAsJsonObject(JsonElement.java:90) ~[minecraft_server.jar:?]
at wn.a(SourceFile:82) ~[minecraft_server.jar:?]
at wn.<init>(SourceFile:48) ~[minecraft_server.jar:?]
at vo.a(SourceFile:915) ~[minecraft_server.jar:?]
at te.<init>(SourceFile:159) ~[minecraft_server.jar:?]
at vo.g(SourceFile:442) ~[minecraft_server.jar:?]
at ud.b(SourceFile:113) ~[minecraft_server.jar:?]
at ud.Y_(SourceFile:65) ~[minecraft_server.jar:?]
at hw.a(SourceFile:238) ~[minecraft_server.jar:?]
at ua.c(SourceFile:171) [minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.w(SourceFile:749) [minecraft_server.jar:?]
at sn.w(SourceFile:357) [minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.v(SourceFile:656) [minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:559) [minecraft_server.jar:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_144]
[19:35:57] [Server thread/INFO]: com.mojang.authlib.GameProfile@1cfc7f7c[id=c766c0bd-82da-365a-a441-f36ec7d78c0a,name=Syrud,properties={},legacy=false] (/37.189.59.244:53446) lost connection: Disconnected