Cannot create a SiteCollection "/" by PowerShell

Hello,

For a deployment I try to play a powerShell script to create 2 sitesCollections :

SC1 => Root => "/" => New-SPSite  -Url $urlTemplate1 -OwnerAlias $owner -Template "Template1" -Name "Template1" => Error.

SC2 => Root => "/SC2" => New-SPSite  -Url $urlTemplate2 -OwnerAlias $owner -Template "Template2" -Name "Template2" => Valid

The first creation returns a security error :

New-SPSite : <nativehr>0x80070005</nativehr><nativestack>OWSSVR.DLL: (unresolve
d symbol, module offset=000000000000F4A4) at 0x000007FEE6B4F4A4
OWSSVR.DLL: (unresolved symbol, module offset=00000000000C86B1) at 0x000007FEE6
C086B1
mscorwks.dll: (unresolved symbol, module offset=00000000002BE497) at 0x000007FE
F5E3E497.................

+ New-SPSite <<<<   -Url $urlTemplate1 -OwnerAlias $owner -Template "Template1#0" -Name "Template1"
    + CategoryInfo          : InvalidData: (Microsoft.Share...SPCmdletNewSite:
   SPCmdletNewSite) [New-SPSite], UnauthorizedAccessException
    + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletNewSite

$owner is Local Administrator.

Have an idea ? on internet there isn't a lot of case like that...

It works when I try to create the Site Collection by Central Admin....

Thank you.