Invalid data or no response: The receiving plugin may not be activated at:

I'm trying to set up the MHMCS Provisioning on a multisite installation and keep getting an error message that has been mentioned on these forums before:

Invalid data or no response: The receiving plugin may not be activated at: http://domain.com

My setup is:
Linode
LEMP
PHP-FPM
WHMCS directory: domain.com/clients
Template: tried both portal and default (though if it needs to be portal that should be mentioned in the usage instructions)
Server: double-checked hostname (domain.com) and IP address. The "Login to Wordpress" button works as it should

Any help would be appreciated as my main reason for getting a WPMU Dev membership was for the WHMCS plugins. Thanks.