did you do any edits within .htaccess?
did you put in the address?
If your oxwall site is at
www.mysite.com and you have the forum plugin then the site will try to got the oxwall forum plugin. So if you installed for example phpbb in
www.mysite.com/forum it MAY conflict with oxwall. Consider changing the subdir to forums.
You can also use the "link to a page in the site" and after your URL enter forums/ and in the menu it will have a button that will take you to
http://www.mysite.com/forums/ and your forum app (phpbb) will open.
I tried it in my live site where I have a development subfolder and using the local page option i was able to get things moving.
That should be a quick fix. It will still need looked at.