Comments not working after Drupal upgrade

Hi

After upgrading from 4.5.x, I have now an almost functional 4.7.6 site, but comments are not working: whenever a user tries to post a comment, he/she gets an 'Access denied. You are not authorized to access this page' error.

Please note that:

- Yes: the core module is enabled,
- Yes: the permissions are set up in access control
- Yes: commenting is enabled in every post, so the comments form appears at the bottom,

Can somebody suggest a solution for this? Maybe a filesystem permissions issue? Where else can I check?

Thanks in advance for any assistance
--
Albert