phpbb works fine on nginx.
See also https://github.com/phpbb/phpbb3/blob/de ... ample.conf
for sample configuration.
It sounds like the domain you entered when you installed the board is not the domain you are using to access the board (and this has nothing to do with your choice of web server). Assuming forum.notinprod.dev is the domain you are using for accessing your board, when you installed it that's what you should have entered instead of forum.mydomain.com.
Edit: reading your post again it looks like the domains you specified have nothing to do with the issue. Did you make up both domains and is the actual domain you are using not either of the ones you did specify?
The issue seems to be incorrect passing of environment variables to php. You should start with the config file I linked to above to see if it would fix the issue. If it does not, check whether other php scripts work correctly.