If you have the forum hosted with a hosting company and not on one of your home pc's ignore all of this, it doesn't apply to you.
Is the forum being hosted on a pc behind the same router as the one you were using while trying to access it?
In other words, are you hosting the forum yourself on one of your pc's?
If this is the case, don't use the domain name...it may not work.
This is a common issue when hosting from a home pc.
You might have to use the IP of that pc on your LAN in which your forum is hosted on instead of the domain name.
This would not
be your external IP that the rest of the world would connect to...not
the IP you have your domain name pointing to.
This would be the IP that is assigned to that pc on your LAN, which is different. (Check your router to see what it is.)
I run a web server on my home pc for various reasons. My daughter can't connect to it using the domain name I have set up for it. She must use this pc's LAN IP address instead.
It would look similar this for you:http://192.168.1.48/vitation/index.php
(replace the LAN IP with the one for that pc which is hosting the forum)
This isn't an IP the rest of the world can connect to...only those on my LAN, those on my side of the router. The rest of the world on the other side of my router can access through my domain name.
See if this works for you.