The problem was solved (see att. pic).

Please, accept the report of the bug

File:
/hooks/gallery_attach_forums_b5837a552916d3be8d5333452a37ae85.php (or similar)

Find:

$category = array_merge( $category['_latestImage'], $category );
$string .= $this->registry->output->getTemplate('gallery_external')->$

Replace with:

if ($category['_totalImages']) {
$category = array_merge( $category['_latestImage'], $category );
$string .= $this->registry->output->getTemplate('gallery_external')->$
}

In theory this happened because there is category in the Gallery but there are no pictures in it.

Please, check if there are more places in your code where this problem may arise.

http://community.invisionpower.com/resources/bugs.html?do=add&category=196

Please report it in the correct area, feedback is not a bug tracker.