I should have added that if a Nabble forum is hidden behind a password protected site then it is not normally required that users register or login in to the Nabble application. The default settings that allow "Anyone" to view and post should be fine.
If you still have a problem, then you need to explain more, including reporting the operating systems and browsers which exhibits these issues.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.