The logo in the top left of every page uses the site domain as its link. It does not add the subfolder to the URL, meaning that if you right click the link to open it, or open in a new tab or window it leads to the root domain and not the forum subfolder.
What is your site URL ?
The ember router catches
/ and properly send you to
/forum. Of course that breaks on new pages/tabs.
There is a customization option (script) to override the logo click, so you can make it go to
/forum or even