Unable to login to "discover"

The “discover” theme was working. Although the website seems to be working OK – I’m unable to logon. I get an error message when attempting to logon:

Warning: Cannot modify header information – headers already sent by (output started at /websites/123reg/LinuxPackage22/jo/hn/so/johnsonfellowes.co.uk/public_html/blog/wp-content/themes/discover/functions.php:46) in /websites/123reg/LinuxPackage22/jo/hn/so/johnsonfellowes.co.uk/public_html/blog/wp-includes/pluggable.php on line 876