Do you have a URL that we can look at? It might help. I will have a stab in the dark :)
It sounds like you have the create topic/reply buttons displayed even though the user is not able to create the content (not logged in and/or no permissions)
When the user clicks on one of these buttons they will get the "access denied" page with the login form. The login form remembers the previous url. In this case the previous url is /content/action (the url used for creating objects). So when the user successfully logs in they are redirected to the previous URL - /content/action but now the form POST variable are no longer present and the code doesn't know what action it should take and the error message is displayed.
Actually there is a similar issue on the eZ forums. (This <b>is</b> Gregs bug)
What I usually do is either disable the buttons or replace them with a message - "you must be logged in to post" if the current user is not logged in and/or doesn't have permissions to create a topic or reply.
The idea is to allow Anonymous users to be able to comment on Forums and Blogs without having to login.. Which was doable on eZ 2.2 but can't get it to work on 3.9
I've even assigned complete access rights to Anonymous users and they still can't post replies nor comments on forums or blogs.
The {$node.object.can_create} comes back empty or in other words the template can't properly grab the Anonymous permissions.