The allowed_iframes setting is unrelated to the 403 error; you’re getting it for a different reason.
If an iframe src is not permitted by Discourse it won’t be embedded, e.g. see below:
iframe here
iframe here
This oddity and the “drafts offline” warning in your computer make me wonder if you have something incompatible in the way.
For example, you’re using an Apache proxy in front of your instance - it might be rejecting the request.