Since yesterday I can no longer log in to the dashboard of my WordPress sites. I can log in the main site. But when I go from the main site to the dashboard of another site of the multisite setup, I get a login screen. When I log in I get the error:
ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.
I am using WordPress 3.8. It runs behind Varnish, maybe that is causing it. I have already overwritten the wp-login.php file. I have network disabled all the plugins. The only thing that temporarily helped was adding the following to wp-config.php:
I could then go to the dashboard of the other site without needing to enter my log in details. However when I commented the above lines the same error occurred.
BTW I do not have cookies disabled. I tried it in IE, Firefox and Chrome. All the same issue.
Does anybody know a solution? Thanks.