This is just FYI - I've fixed the issue on my site.
site.com/login causes error;
Warning: Cannot modify header information - headers already sent by (output started at /home/commerci/public_html/wp-content/themes/blogs-mu/page-template-custom-login.php:7) in /home/commerci/public_html/wp-includes/pluggable.php on line 881
Error was actually correct ( I assumed it was one of my mods so chaced them first:grinning:
page-template-custom-login.php line 7 white space before <?php
Template Name: Custom Login
<------DELETE THIS LINE FEED