Problem with connection and cache

Hi,

when users try to connect on the page, they have an error message ” you cannot login, this page has probably been put on cache” (translated approximatively from French).

I can manage to login after second or third attempt, but the members are usually not very tech savy and it puts them off, when they can’t login on the first attempt and just give up.

I’ve purged my cache with WP super cache and I’ve used basic settings for this plugin.

Anything I’m supposed to do to avoid having this message ?

Thanks for your help.

The page I need help with: [log in to see the link]