check your system date/time.
i assume you mean florian's component and not the official client? it works fine with the current version of foobar but it does require the correct date to work. last.fm would obviously reject scrobbles from the future. you'll lose any cached scrobbles but you'll need to delete your old config file. hold down shift, click on file>browse configuration folder. now close foobar and open the configuration folder in explorer. delete foo_audioscrobbler.dll.cfg. start foobar again and make sure you go back in and set your last.fm username/password in the preferences.
In the configuration-folder, not components.