oAuth does not respect custom server settings

I'm running phpBB behind a reverse proxy on the internal IP (192.168.0.123). Even though I told the board to use custom server url settings (https://forum.ichbinein.org), when I use oAuth, the callback URL contains the url-encoded internal IP address:

https://www.facebook.com/dialog/oauth?type=web_server&client_id=xxxxxxxxxxxxxxx&redirect_uri=https%3A%2F%2F192.168.0.123%2Fucp.php%3 ...

I suspect that the server settings are not carried over to the oAuth system.