THANK YOU for posting this! I was able to solve the issue thanks to your post. I'll post the fix here for anyone who stumbles upon this post in the future.
Find the following file admin\applications\core\modules_admin\mycp\dashboard.php
Change line 377 to
$online = $this->DB->buildAndFetch( array( 'select' => 'count(*) as fixme', 'from' => 'sessions', 'where' => "running_time > {$time}" ) );
Change line 379 to
$stats['active_users'] = $online['fixme'];
Of course you don't have to use the word "fixme". You can use any word so long as you don't use "rows"!. Thanks!