Repetitive Log out

Dear all,

When I’m logged in to my admin page, I get automatically logged out everytime I click on a tab or on a link in my dashboard. When I relog in, I’m then directed to the page I wanted to go.
To be clearer by the way of an example, after having logged in for the first time, I click on “Post” for instance, I’m then asked to log in again, and then I’m directed (correctly) to the “Post” page. If I want to create a new post, I then click on “Add new”, and then again, I’m asked to log in, but then I end up in the correct new blank page to write a new post, and so forth and so on.
I run on wordpress 3.8.5 Could you please tell me how to fix this issue?

Thanks a lot.