Error 403 at create user setup step

I have installed InvoicePlane into a directory on a Namecheap hosted domain. The setup process breaks when it comes time to create the user though, always ended up at a 403 forbidden error page when I click submit, regardless of how many or few boxes are filled in.

I’ve been through the other suggestions on the forums that I came across - recreating a new blank database (no change) and manually injecting a user and then resetting the password (it never sends me an email to reset the password). I receive no other errors up to this point (a clean suite of green ticks).

Exact error message:
Logs (SSL): (domain redacted)