when logging in it asks to log in again

Since about 1 month we have this problem:

Normally non-admin members go to membership-login/ and log in. They are then directed to a page to select access to other pages with protected content.

But when they select any of the pages it appears that they must log in again.

I have to flush the site cache in the wp-fastest cache plugin

When emptying the cache they can enter without problem, but after a few hours this problem occurs again.

I have added https://creaccion.co/membership-login/ as an exclusion in the cache plugin, but the problem persists.

Thank you.

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