Impossible to log out

Hi

Once I log in in the web, it is impossible to log out: thoug I click on logout several times, I stay connected.
I have checked the WP Supercache settings just in case it was the issue, but I have configured it as it is indicated in https://simple-membership-plugin.com/wp-super-cache-compatibility-simple-membership/

This issue happens in three different browsers.

Thanks in advance

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