Have you tried to recreate your external post island again?
-Moving to post_island_12.php creates the same error.

Have you updated the files in the cache_builders/ directory to your server? (cache_builders/ ONLY. not cache_builders/custom/)
-Yes, have reupload again. Same error.

Have you cleared your forum cache since your upgrade?
-Yes, also have re-cleared. Same error.

A--> Have already tried that, but forgot to backup before uploading.

B--> Not apply

C--> I have allowed the upgrader to update the file automatically.

Would uploading 7.5.9 generic.php helps? Since I did backup before upgrading. But did not do it before re-uploading that file.

Btw, it does post all the topic/message, just that the error appears at the beginning.
When posted, it is above the "Name" of the post island.

"Notice: Undefined variable: config in C:\Web\etplanet\forums\languages\english\generic.php on line 117
Notice: Undefined variable: config in C:\Web\etplanet\forums\languages\english\generic.php on line 186
Latest Post" <-- my name of the post island

Note: I was upgrading from 7.5.9 to 7.6.0

Also, the post island is there to show recent post. No other codes are added/used.