If you are running PS 3+ add -LiteralPath switch to your rename:
Rename-Item -LiteralPath $folder.FullName $newname
otherwise use Move-Item
Move-Item -LiteralPath $folder.FullName $newname
Powershell doesn't like square brackets in filenames, more in the following post:
This became an issue with V2 when they added the square brackets to the wildcard character set to support "blobbing".
From get-help about_wildcards
:
Windows PowerShell supports the following wildcard characters.
Wildcard Description Example Match No match
Matches zero or a* A, ag, Apple banana more characters
? Matches exactly ?n an, in, on ran one character in the specified position
[ ] Matches a range [a-l]ook book, cook, look took of characters
[ ] Matches specified [bc]ook book, cook hook characters
[
and ]
are special characters.