Login, preview and media errors

A few weeks ago, I started having to login to WordPress through /wp-login.php instead of /wp-admin. That’s not a problem, but since this started occurring I can no longer preview my posts or upload certain media (particularly feature images).

I usually get errors like this that pop up all over the place when I try to preview etc:

Warning: Cannot modify header information – headers already sent by (output started at /home2/editor/public_html/wp-content/themes/magazino/functions.php:3) in /home2/editor/public_html/wp-includes/pluggable.php on line 1173

Usually with various numbers at the end though or different links.

Does anybody know the fix for this? Using Magazino theme, site is: glasscity.ca. Thanks.