If SharePoint Designer doesn't work, maybe you shoult try doing this with PowerShell intead?
# Add PS Snapin if not loaded
$snapin = Get-PSSnapin | Where-Object {$_.Name -eq 'Microsoft.SharePoint.Powershell'}
if ($snapin -eq $null)
{
Write-Host "Loading SharePoint Powershell Snapin"
Add-PSSnapin "Microsoft.SharePoint.Powershell" -EA SilentlyContinue
}
#Iterate all webs and print List URL
$SPWebApp = Get-SPWebApplication "http://mydomain.com/"
foreach ($SPSite in $SPWebApp.Sites)
{
if ($SPSite -ne $null)
{
foreach ($SPWeb in $SPSite.AllWebs)
{
if ($SPWeb.url.Contains("http://mydomain.com/sites/myweb"))
{
$SPWeb.Name = "myNewName"
$SPWeb.Title = "My New Name"
$SPWeb.Update()
if ($SPWeb -ne $null)
{
$SPWeb.Dispose()
}
}
}
}
if ($SPSite -ne $null)
{
$SPSite.Dispose()
}
}