Cannot login into my subdomain. Get an error message "enable cookies"

Hi all,

New to wordpress, I have tried to solve the problem by going through a number of the feeds here to no avail.

My site is a MU site with a main domain that maps to a second domain. I cannot login into the subdomain as my admin account (as well as any other accounts) brings up the error message:

“ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.”

This occurred after a hacker had gained access to the website, resulting in us getting black listed. We installed Sucuri to clean and secure the site which it did as well as de-blacklist us. We have the Sucuri web montior installed as well as the Cloud Proxy firewall which caused a lot of problems as it prevented me from actually getting to the login page. Now that has been resolved I found that I was getting the error message above.

To try and solve this I followed the following advice recommended by other threads:

– cleared browser cache and history and data
– deactivated all the network plugins and themes to try and resolve the issue. Did not work so are all activated again.
– adjusted the code in the wp-config.php file

I have seen a number of other things but none of them relate to the exact issue I am having.

Any help would be most appreciated.

Kind Regards,