After upgrading to 2.5.5, the top right link that links & redirects to the client site is not working.

The link is there, and generates the correct URL, but redirect never happens and we're stuck with the blank page:

<http://www.clientsite.com/?URL=http://www.clientsite.com/>

What could be causing this issue?

Thanks everyone!

P.S. Unfortunately none of the solutions at the related [Control Panel (CP) Link Button is not working](https://expressionengine.stackexchange.com/questions/2403/control-panel-cp-link-button-is-not-working) issue worked.