I have this "403 Forbidden" page that appears after clicking on the "create topic" button with this error message "You don't have permission to access this resource.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request."

where can this error come from?

A 403 message is your server denying access somewhere. You would need to contact your hosting company to ascertain where the issue is occuring. Check for things like mod_security

so it doesn't work anymore thank you for your answer