I get the same problem on unraid 6.9.1 on both a forge and spigot server.
[12Mar2021 17:50:08.152] [main/FATAL] [net.minecraft.server.Main/]: Failed to start the minecraft server
java.lang.RuntimeException: java.nio.file.FileSystemException: .: Operation not permitted
at net.minecraft.world.storage.SaveFormat.<init>(SaveFormat.java:77) ~[?:?]
at net.minecraft.world.storage.SaveFormat.func_237269_a_(SaveFormat.java:85) ~[?:?]
at net.minecraft.server.Main.main(Main.java:115) ~[?:?]
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?]
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:?]
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
at java.lang.reflect.Method.invoke(Method.java:566) ~[?:?]
at net.minecraftforge.fml.loading.FMLServerLaunchProvider.lambda$launchService$0(FMLServerLaunchProvider.java:51) ~[forge-1.16.5-36.0.35.jar:36.0]
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.0.9.jar:?]
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.0.9.jar:?]
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.0.9.jar:?]
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.0.9.jar:?]
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.0.9.jar:?]
at net.minecraftforge.server.ServerMain$Runner.runLauncher(ServerMain.java:63) [forge-1.16.5-36.0.35.jar:?]
at net.minecraftforge.server.ServerMain$Runner.access$100(ServerMain.java:60) [forge-1.16.5-36.0.35.jar:?]
at net.minecraftforge.server.ServerMain.main(ServerMain.java:57) [forge-1.16.5-36.0.35.jar:?]
Caused by: java.nio.file.FileSystemException: .: Operation not permitted
at sun.nio.fs.UnixException.translateToIOException(UnixException.java:100) ~[?:?]
at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111) ~[?:?]
at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:116) ~[?:?]
at sun.nio.fs.UnixPath.toRealPath(UnixPath.java:860) ~[?:?]
at net.minecraft.world.storage.SaveFormat.<init>(SaveFormat.java:75) ~[?:?]
... 15 more