Prestashop 1.6 logs you out after 15 minutes, so the login is being cancelled after 15 minutes of inactivity in the PrestaShop admin area.

This 15 minute timeout can be edited and adjusted in the following folder:
classes/controller/AdminController .php

At the line 2288, or if it changed do a string search for this line:
if ($this->context->cookie->last_activity + 900 < time()) {

Changing  the 900 (seconds) to 3600 (seconds) extends the 15 min period to 60 minutes.