Did you check the log ?
Permission problems most of the time produce a clear message.
You could also try the Set-VIPermission cmdlet on a parent folder with the -Propagate parameter.
That way you should be able to assign you the proper permissions on the folder.
Make sure you select the correct value for the -Role parameter.
If it doesn't work could you do a Get-VIPermission on the folder ?
And also a Get-VIPrivilege -Role with the roles you get back from the previous cmdlet ?
That could perhaps allow us to see why it goes wrong.
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Blog: LucD notes
Twitter: lucd22
Blog: lucd.info Twitter: @LucD22 Co-author PowerCLI Reference