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I'm having the same problem. Unable to solve it so far.
@johnmiller @maburns @zradick did you manage to get past this problem, and if so, how?
Thanks for any advice!

OK so I was finally able to track it down fix it. Some short notes below, maybe it helps anyone else running into the same problem.
Thanks to @mark-robustelli and @dan for their inputs.

1. I accessed FusionAuth directly on port 9011, completely bypassing the reverse proxy
2. In the CORS filter config, I activated "Debug enabled" which in the system logs showed me java.lang.IllegalArgumentException: Illegal character in authority at index 7 and java.lang.NullPointerException: Cannot read field "actionURI" because "actionInvocation" is null
3. Going from there, I completely disabled the CORS filter
4. This immediately got rid of the 403 and everything worked again, accessing FA using the reverse proxy
5. The FA UI told me that I'm missing two headers in my reverse proxy config: X-Forwarded-Proto and X-Forwarded-Port, which I then configured (https/443 in my case)
6. After that I enabled the CORS filter again (all default values) and since, everything is working correctly, like before