Error con el plugin Wp-Supercache

Estoy tratando de configurar este plugin y al hacer el test de cache, me aparece el siguiente error:

Cache Tester
Test your cached website by clicking the test button below.
Fetching http://opositaonline.com/ to prime cache: OK
Fetching first copy of http://opositaonline.com/: OK (1.html)
Fetching second copy of http://opositaonline.com/: OK (2.html)

ERROR!!!!

The pages do not match! Timestamps differ or were not found!
Things you can do:
Load your homepage in a logged out browser, check the timestamp at the end of the html source. Load the page again and compare the timestamp. Caching is working if the timestamps match.
Enable logging on the Debug page here. That should help you track down the problem.
You should check Page 1 and Page 2 above for errors. Your local server configuration may not allow your website to access itself.

¿Cuál es el motivo? Sin esto, el plugin no hace nada y es como si no tuviera cache alguna...

URL del sitio: Contenido solo visible a usuarios registrados