I can not access
admin/people/permissions, even the link to permissions in the backend-menu is not visible. But I am logged in as admin! So what can be the problem here?
Your administrative role probably lacks the "administer permissions" permission.
If you have Drush installed:
$ drush role-add-perm 'administrator' 'administer permissions'
You may also want to make sure your site still defines the 'administrator' role, since occasionally developers may manually delete the default roles from the DB (really not a good idea unless you know what you're doing). If you can't view available roles you can run this Drush command:
$ drush role-list
If it's not there, then you may have to create a new role to replace the admin role that was deleted and assign the role to yourself, and give your new role the 'administer permissions' permission:
$ drush role-create 'administrator' $ drush user-add-role 'administrator' 'your username' $ drush role-add-perm 'administrator' 'administer permissions'
If for some reason you don't have terminal access to your server, then you may need to download and enable the Devel module and PHP filter, then write a script that you can paste and run in /devel/php, but that should really be your last resort. https://drupal.stackexchange.com/a/56495/72084
p.s.: I believe Drupal gives user 1 the administrator role and completely unrestricted access by default. So if you are logged in as someone other than user 1, and/or if the administrator role has been deleted, that may explain how this problem came about. https://www.drupaleasy.com/quicktips/beginner-tip-what-special-about-drupal-user-1