When using Active Directory security on the front-end of your site, adding/editing a forum topic fails.
This is due to the fact that the se:cmsupdate macro being used automatically tries to retrieve the user's username and password.

These credentials are needed to be able to make a webservice call to the cms manager. But when Active Directory security is in place, this is not possible since the AD Membership Provider doesn't allow password retrievel.

The se:cmsupdate macro exposes two parameters (username and password) to specify fixed credentials. So, the first thing that must be done, is change the SaveTopic() translation and specify fixed credentials within each instance of se:cmsupdate.

The credentials specified should either be the username and password of an Smartsite account (when using Smartsite security for the cms manager) or the username and password of an domain account (when using Active Directory security through basic authentication for the cms manager). In both cases, the specified account should have sufficient access rights within the manager environment.

However, specifying the appropriate credentials only makes sure the authentication of the webservice call succeeds. Without additional steps, every forum topic change will be executed on behalf of the specified (and fixed) credentials.

Therefore, the cms:update macro (since iXperion 1.2) passes the current logged on user's username as hidden parameter when a fixed username has been specified as macro parameter, so any cms action (in this case add/edit forum topic) is executed on behalf of this user (and not the fixed credentials specified within the se:cmsupdate macro).

Hence, the second step include determining/creating an appropriate (Smartsite or Domain) account and giving it sufficient access to the cms manager, including contenttypes, content items, etc.