NTFS permision via powershell - powershell

Question

I try to set NTFS permissions for domainlocal groups via powershell.
therefore, I followed this tutorial:
https://technet.microsoft.com/en-us/library/ff730951.aspx
I adapt that code to create the code snippet below:
function set_NTFS_permissions ($group,$folder){
$rule = "" # Just to make sure its empty
$user_read = "gfo\"+$group+"_read"
#$read_permissions = #("ReadAndExecute","Synchronize")
$read_permissions = "ReadAndExecute,Synchronize"
$user_write = "gfo\"+$group+"_edit"
$write_permissions = "DeleteSubdirectoriesAndFiles,Write,ReadAndExecute,Synchronize"
$acl = get-acl $folder # Calls for current permissions
$acl.SetAccessRuleProtection($True,$True) # Protects inherit permissions (superior | inferior)
$rule = New-Object System.Security.AccessControl.FileSystemAccessRule($user_read,$read_permissions,'ContainerInherit,ObjectInherit','None','Allow')
$acl.AddAccessRule($rule)
Set-Acl $folder $acl
$rule = New-Object System.Security.AccessControl.FileSystemAccessRule($user_write,$write_permissions,'ContainerInherit,ObjectInherit','None','Allow')
$acl.AddAccessRule($rule)
Set-Acl $folder $acl
}
$folder = '\\domain.local\path\path\path'
$group = 'DL_fs_path'
set_NTFS_permissions $group $folder
Get-Acl $folder | Format-List
This one works just fine.
My problem starts, when I use that function in another funtion.
There, I create several folders and groups (which works) and then I want to permit the groups to the folders.
To do this, I call the above funktion with some variables (which are transfered correctly ... Write-host them just before passing them to the function and then again at the beginning of the function.)
Ausnahme beim Aufrufen von "AddAccessRule" mit 1 Argument(en): "Some or all identity references could not be translated."
In C:\Scripts\filestruct.ps1:20 Zeichen:5
+ $acl.AddAccessRule($rule)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [], MethodInvocationException
+ FullyQualifiedErrorId : IdentityNotMappedException
Has anyone an idea, why it works when statics are passed, but not with variables?