After migrating from CraftBukkit to Spigot, starting Spigot for the first time I see in the early steps this warning followed by an exception:

[10:02:37] [Server thread/WARN]: Unable to read file banned-ips.json, backing it up to {0}.backup and creating new copy.
com.google.gson.JsonSyntaxException: Expected a com.google.gson.JsonArray but was com.google.gson.JsonPrimitive
        at com.google.gson.internal.bind.TypeAdapters$35$1.read(TypeAdapters.java:911) ~[gson-2.8.8.jar:?]
        at com.google.gson.Gson.fromJson(Gson.java:932) ~[gson-2.8.8.jar:?]
        at com.google.gson.Gson.fromJson(Gson.java:870) ~[gson-2.8.8.jar:?]
        at net.minecraft.server.players.JsonList.f(JsonList.java:163) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3442-Spigot-699290c-62d9762]
        at net.minecraft.server.dedicated.DedicatedPlayerList.y(SourceFile:76) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3442-Spigot-699290c-62d9762]
        at net.minecraft.server.dedicated.DedicatedPlayerList.<init>(SourceFile:25) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3442-Spigot-699290c-62d9762]
        at net.minecraft.server.dedicated.DedicatedServer.e(DedicatedServer.java:194) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3442-Spigot-699290c-62d9762]
        at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:994) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3442-Spigot-699290c-62d9762]
        at net.minecraft.server.MinecraftServer.lambda$0(MinecraftServer.java:304) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3442-Spigot-699290c-62d9762]
        at java.lang.Thread.run(Thread.java:833) [?:?] 

Looks like you have an error in the code that generates the file banned-ips.json.