Forum Error message in ACP when redirect enabled
 

When I try to make a change in the ACP I get the following error message:

In order to accept POST request originating from this domain, the admin must add this domain to the whitelist.

This all started after I enabled a redirect on my site. I have a redirect from a subdomain to the main domain. If I remove the redirect I don't get the error message. However, I really need the redirect.

How can I solve this? Thank you!