Error When Logging In

After logging in I get this error:

Warning: Cannot modify header information - headers already sent by (output started at /home/content/58/8530558/html/foundersyncnetwork/wp-content/themes/blogs-mu/page-template-custom-login.php:7) in /home/content/58/8530558/html/foundersyncnetwork/wp-includes/pluggable.php on line 881

Please advise