I've had versions 6.5.4 and now (I think) 7.2.
I've never seen the "apply" button after a rescan, but then again, I've not rescanned my library after the update. It did that automatically after the update and I've not added anything to the library to warrant a rescan.
Just curious: If you click rescan I do believe the screen just flashes (by design). However if you immediately go back to the home page to view your library, it should say something like "your library is being rescanned" (the message is version dependant, and I'm not sure what 7.0 specifically says). There are no big fancy bells and whistles to indicate the scan is still running.
Um... Just for grins rklein, update to the newest
version 7.2.1 as they advertise "Numerous bug fixes", and "Squeezebox firmware updates to fix a number of issues".
Other than that, I'm at a loss.

Bob