can forums be public while the project is private?
or is there any other way to expose to "external" users forums for discussions even if they are not part ot the openproject users?

Aren't you using a setup just like this here?

Hi Jonathan,

forums are activated as a module in each project. You cannot make a single module of a project public. That is why you need to make the whole project public in order to make the forum publicly available. 

Best,

Alexander

thanks for the answer, my idea it was to provide public forum but keep versions, backlogs and all project stuff private, but seems that is not possible.

more like, 
provide a way that users can report bugs or ask for help/clarifications and me (as developer) can review it raise a bug or provide the info to the user.
i though forums of a project can be public and keep the project stuff like versions, calendars, backlogs, etc, etc. private

Hello Jonathan,

understood, thank you!
There is an enterprise feature that allows you to share just certain work packages of a project with another user. [1]
Alternatively, it is at least possible to permit users to access certain modules in your project by defining roles accordingly. OpenProject allows you to define a fine grained set of permissions for each role. You can also set up workflows that just allow a user to e.g. only open a new bug report. For example, you could create a role "user" which has very limited permissions to create new work package and has no access to the backlog module, or calendar etc.

I encourage you check out the user guide on workflows and roles for more information on this:
https://www.openproject.org/docs/system-admin-guide/users-permissions/roles-permissions/#roles-and-permissions
https://www.openproject.org/docs/system-admin-guide/manage-work-packages/work-package-workflows/#manage-work-package-workflows

Further, you could also duplicate a work package to another project and set a relation between both. [2] There is even a duplicates / duplicated by relation you can make use of.

Best,

Alexander

[1] https://www.openproject.org/docs/user-guide/work-packages/share-work-packages/#share-work-packages-enterprise-add-on
[2] https://www.openproject.org/docs/user-guide/work-packages/work-package-relations-hierarchies/#work-package-relations-and-hierarchies