Anybody found any solution on this, since this issue is still persist with Plesk 10.4.4
Disable firewall for sometime and try to recreate an account. If this issue still persists then I would suggest you to submit a ticket with WHMCS support and see if they provide you any solution.
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Generally this happens if the connection is blocked between WHMCS server and your Plesk server. If you face this problem again, check the logs from Plesk server and if possible try to access your Plesk server from WHMCS server via SSH (if it is Linux). Definitely it's a sporadic connection problem between two servers.