This happens when downloading the LuckPerms Placeholder from PlaceholderAPI cloud.
when reloading, I get an internal error (in chat) and following error in console:

Only happens with LuckPerms (so far)

Using LuckPerms for bungee on a Waterfall server.
Using Paper.jar (1.15.2)

org.bukkit.command.CommandException: Unhandled exception executing command 'papi' in plugin PlaceholderAPI v2.10.4
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:47) ~[patched_1.15.2.jar:git-Paper-112]
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:159) ~[patched_1.15.2.jar:git-Paper-112]
at org.bukkit.craftbukkit.v1_15_R1.CraftServer.dispatchCommand(CraftServer.java:742) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.PlayerConnection.handleCommand(PlayerConnection.java:1821) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.PlayerConnection.a(PlayerConnection.java:1629) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.PacketPlayInChat.a(PacketPlayInChat.java:47) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.PacketPlayInChat.a(PacketPlayInChat.java:5) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.PlayerConnectionUtils.lambda$ensureMainThread$0(PlayerConnectionUtils.java:23) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.TickTask.run(SourceFile:18) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeTask(IAsyncTaskHandler.java:136) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.IAsyncTaskHandlerReentrant.executeTask(SourceFile:23) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeNext(IAsyncTaskHandler.java:109) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.MinecraftServer.ba(MinecraftServer.java:1038) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.MinecraftServer.executeNext(MinecraftServer.java:1031) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeAll(IAsyncTaskHandler.java:95) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.MinecraftServer.a(MinecraftServer.java:1172) ~[patched_1.15.2.jar:git-Paper-112]
at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:934) ~[patched_1.15.2.jar:git-Paper-112]
at java.lang.Thread.run(Thread.java:834) [?:?]
Caused by: java.lang.NoClassDefFoundError: net/luckperms/api/LuckPerms
at me.lucko.luckperms.placeholders.LuckPermsExpansion.canRegister(LuckPermsExpansion.java:48) ~[?:?]
at me.clip.placeholderapi.expansion.ExpansionManager.registerExpansion(ExpansionManager.java:102) ~[?:?]
at me.clip.placeholderapi.expansion.ExpansionManager.registerAllExpansions(ExpansionManager.java:154) ~[?:?]
at me.clip.placeholderapi.PlaceholderAPIPlugin.reloadConf(PlaceholderAPIPlugin.java:176) ~[?:?]
at me.clip.placeholderapi.commands.PlaceholderAPICommands.onCommand(PlaceholderAPICommands.java:226) ~[?:?]
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:45) ~[patched_1.15.2.jar:git-Paper-112]
... 17 more
Caused by: java.lang.ClassNotFoundException: net.luckperms.api.LuckPerms
at java.net.URLClassLoader.findClass(URLClassLoader.java:471) ~[?:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:588) ~[?:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:521) ~[?:?]
at me.lucko.luckperms.placeholders.LuckPermsExpansion.canRegister(LuckPermsExpansion.java:48) ~[?:?]
at me.clip.placeholderapi.expansion.ExpansionManager.registerExpansion(ExpansionManager.java:102) ~[?:?]
at me.clip.placeholderapi.expansion.ExpansionManager.registerAllExpansions(ExpansionManager.java:154) ~[?:?]
at me.clip.placeholderapi.PlaceholderAPIPlugin.reloadConf(PlaceholderAPIPlugin.java:176) ~[?:?]
at me.clip.placeholderapi.commands.PlaceholderAPICommands.onCommand(PlaceholderAPICommands.java:226) ~[?:?]
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:45) ~[patched_1.15.2.jar:git-Paper-112]
... 17 more

Fixed: I forgot to install LuckPerms on the spigot server as well, this meant PlaceHolderAPI didn't know what the expansion was used for. hence the error