Problem accessing Google calendar

I have a Samsung SIII phone. I have synced my Google calendar, contacts etc successfully and for most of the time everything works ok. Now and again however, it all goes pear-shaped and I am unable to access my calendar. I am taken to a sign-on page and when I try to input my password the keyboard appears momentarily and then disappears just as quickly! Meanwhile, the progress bar at the top of the screen flicks across and then back again constantly. I have tried emptying the cache and removing/reinstalling the account all to no avail.

via Forums de téléphone cellulaire (en français) http://cellphoneforums.net/fr/samsung/t365503-problem-accessing-google-calendar.html