You can fix this yourself. The Gadget points to an old, defunct link. First close down the gadget if it is running.

(1) Go to:
C:\Users\YOURLOGIN\AppData\Local\Microsoft\Windows Sidebar\Gadgets\MinerStats.gadget\js

(2) Edit main.js with a text editor.

(3) Find the line that begins with: var SlushPoolAPI ... and edit it to read: var SlushPoolAPI = 'https://mining.bitcoin.cz/accounts/profile/json/';

(4) Restart gadget. Enter your Slush API token. Should now work! Tips appreciated, BTC address below!