SSO site name The site name must conform to the following requirements:

The following ASCII characters are allowed: A-Z, a-z, 0-9, comma (,), dot (.), question mark (?), dash (-), underscore (_), plus sign (+) or equals sign (=).
The domain name must not exceed 63 characters

Site name for the source vCenter SSO needs to be updated because it was originally set to 'local' by default.