1.6.11 problem

[ZiNgA BuRgA]

Maybe open up index.php, and just below the <?php line, add the following:

PHP Code:

ini_set('display_errors', true); error_reporting(E_ALL);

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I don't know whether this has any effect, as I know that MyBB does something with error handling.
A 500 error most likely indicates a PHP error so something should theoretically show.

Another problematic area other than templates may be a corrupt cache/xthreads_evalcache.php file.  Try deleting that file.