Relative redirects are technically not defined in HTTP/1.1, so it’s cleaner to not be returning those (Piwik currently does, e.g. in redirecting / to /index.php?..). Instead, use absolute URLs. Thanks.
“According to the HTTP protocol, the Location header must contain an absolute URI. When redirecting from one page to another within the same site, it is a common mistake to use a relative URI. As a result most browsers tolerate relative URIs in the Location header, but some browsers display a warning to the end user.”