I am only advising this because it worked for me with a custom template. Note: I have not customized the forum from the default layout.
- First, switch your template to Uni and see if you can access the forum. If you can, compile the language again and then proceed to the next step.
- Via FTP, navigate to the modules/boonex/forum/layout directory. Your should see 2 folder in there with your template name. We will use the example of template name 'blue'. So you should see blue and blue_en. Make a backup of these 2 folders to your local machine.
- Next, download JUST the uni folder from the same directory to your local machine.
- After you have made backup copies of your template folders (ex. blue and blue_en). Delete them from the modules/boonex/forum/layout directory
- Rename the uni folder that you downloaded to your local machine to your template name (ex. blue)
- Edit the params.php file and replace the the words 'uni' to match your template name. (ex. blue)
- Re-upload that folder that you just renamed to the modules/boonex/forum/layout directory
- Change your template BACK to your custom template name in the Admin Panel.
- Access the Forum via Admin Panel > Modules - Recompile the language from the Manage Forum tab.
** If you have caching turned on, you will need to clear the cache when you switch templates.
Hope this helps.