If you're reading this in 2024 and still having issues even after implementing all the suggestions here, I just spent a ton of time trying to get to the bottom of this and managed to find a solution as my logs and Sentry were completely clogged.

Short answer: You need to update your nginx.conf file to rewrite the host if it's coming from a fixed IP.

Full write up here: https://diogofreire.btw.so/fixing-the-damn-allowed_hosts-issue-with-amazon-elastic-beanstalk-and-django