I recently changed the version of PHP used on my server from 4 to 5.2. As soon as the change was reflected, I started getting a list of errors both within WP Admin and on the blog page. Whether this is coincidence or not, I don't know, but I did manage to fix it.
By editing the "main.php" file of Mantra and going down to line 24 (indicated in the error message), I "told" it that the value was an array. In other words, change:
$optionsMantra = array_merge($mantra_defaults, $optionsMantra);
$optionsMantra = array_merge($mantra_defaults, (array)$optionsMantra);
Clicking "Save" still causes errors on the Dashboard screen until you refresh, but the blog was fixed immediately.