"Access is denied" error during "New-Item" call in PS - powershell

Question

I am trying to create a folder on a server using powershell. The script looks like this:
$pass = ConvertTo-SecureString "myPW" -AsPlainText -Force
$cred = new-object System.Management.Automation.PSCredential("myUser", $pass)
$session = New-PSSession -ComputerName "localhost" -Credential $cred
Invoke-Command -session $session -ScriptBlock {
New-Item -Path "\\myServer\myShare\" -Name "myFolder" -ItemType directory -Force
}
I'm getting the following error:
Access is denied
+ CategoryInfo : PermissionDenied: (\\myServer\myShare:String) [New-Item], UnauthorizedAccessException
+ FullyQualifiedErrorId : ItemExistsUnauthorizedAccessError,Microsoft.PowerShell.Commands.NewItemCommand
+ PSComputerName : localhost
Access to the path 'myShare' is denied.
+ CategoryInfo : PermissionDenied: (\\myServer\myShare:String) [New-Item], UnauthorizedAccessException
+ FullyQualifiedErrorId : CreateDirectoryUnauthorizedAccessError,Microsoft.PowerShell.Commands.NewItemCommand
+ PSComputerName : localhost
Access to the path 'myFolder' is denied.
+ CategoryInfo : PermissionDenied: (\\myServer\myShare\myFolder:String) [New-Item], UnauthorizedAccessException
+ FullyQualifiedErrorId : CreateDirectoryUnauthorizedAccessError,Microsoft.PowerShell.Commands.NewItemCommand
+ PSComputerName : localhost
At first I thought the message is pretty clear an the user I used is missing rights. But here is the thing: If I go to the share in the Explorer and log in with the same user, I can create a folder without any problems. Besides that, the user is Domain Admin.
What am I doing wrong?

Answer 1

It's a double hop, you're connecting to a PSSession and then trying to access the folder.
As a security measure PowerShell does not allow you to remote to one computer, then remote to another computer from there (even if two of those are the same computer). The exact reasoning is complex and heavily involved in how credentials are used in sessions and I don't fully understand it myself so I won't try to explain but basically it's to prevent the credentials being stolen.
You can do some reading on it (this looks like a good resource) but it would be a LOT simpler to just try to work out another way of doing it. You're only remoting to the local computer so you could for example start another PS process.

Answer 2

If you are just using a PSSession for the alternative credentials to access the share, then using New-PSDrive might be an alternative way to do it:
$pass = ConvertTo-SecureString "myPW" -AsPlainText -Force
$cred = new-object System.Management.Automation.PSCredential("myUser", $pass)
New-PSDrive -Name myShare -PSProvider FileSystem -Root "\\myServer\myShare\" -Credential $cred
New-Item -Path "myShare:\" -Name "myFolder" -ItemType directory -Force