I have been asked by a friend to revamp their WordPress multisite which someone else created for them a long time ago. I signed in to the account and firstly updated it to WP 3.6. Since then every time I try and log in at the normal site address /wp-login.php which the original designer gave them I get the following error message "ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress".
I have tried clearing all cookies, history etc. on multiple browsers which hasn't done anything.
I have also tried logging in at the site address /wp-admin/ and get this message:
Warning: Cannot modify header information - headers already sent by (output started at /home/content/64/9421064/html/wp-content/plugins/twitter/widget_twitter_vjck.php:171) in /home/content/64/9421064/html/wp-includes/pluggable.php on line 896
I only have access to the WordPress account and non of the original source files.
Any help would be much appreciated!