When a new user tries to create a discussion, they are allowed to enter their text and everything, but get a page saying “Forbidden” only when they are trying to “Create discussion”.

That is very discouraging: First off, the issue is easy to remedy by simply logging in, but that is totally non-obvious from the page that opens.

Secondly you lose all the text you entered, which might have taken a lot of work to get right. People might not want to bother to type everything again and just go away without filing their bug report.