Okay, I am having the same problem. The folder and file permissions are accurate.
Folder ‘Forums’ is set to 0755 and the files within it are set to 0644.
I deactivated all of the plugins except bbPress and the 403 error still happens when trying to add a topic to the frontend.
bbPress Version 2.6.9
WordPress Version 6.3
Theme is a child theme of Twenty Nineteen