Have you tried to recreate your external post island again?
Have you updated the files in the cache_builders/ directory to your server? (cache_builders/ ONLY. not cache_builders/custom/)
Have you cleared your forum cache since your upgrade?
If neither of those work for you, here are some further troubleshooting ideas:
A) Make a backup of the file languages\english\generic.php which is already on your server, and then placing a fresh copy of it from your ubbthreads-7-6-0.zip archive in to that folder.
B) If you manually updated your language files by hand:
Double check those two listed line numbers in your language file to confirm that they are formatted correctly -- like the other lines in that file.
C) If you allowed the upgrader to automatically update your language files:
Those two line numbers will be unique to your language files. You can zip up that file and attach it here. Then someone can view it and locate exactly what the problem is caused by.
Ultimately though, since you are using a custom external post island, you might want to open that island up and see if there are any conflicting items within it.
Though, someone else may come along with further ideas. These are what I have as right now.