It appears that UnshortUrl() only follows server-side redirects. Please add support for browser redirects also.
Lately, I see more sites using ‘307 Temporary Redirects’. These are actually browser redirects, not server-side as the ‘30x’ code might imply (https://plus.google.com/+JohnMueller/posts/E4PqAhRJB2V).
For example, the following results in ‘Error:Loop …’:
=UnshortUrl( UrlProperty("express.com", "absolute") )