I'm using Drupal 7 with Varnish. Recently I noticed that the login and sign up blocks are not working for anonymous users. When a user tries to login or create an account via the blocks the user is just redirected to the same page however the new accounts are being created and the user session is also created. Is worth mentioning that if the users tries this via sign in/sign up page they don't have any issues. The problem happens only when using the blocks.
I'm thinking the issue could be related to Varnish. I tried disabling the Varnish module but the issue is still there. Also I tried using the ESI module so that blocks will not be cached but that didn't do anything.
Does anyone has an idea what may be causing this problem?