As you may know Joomla 3.5.1 was released a few days ago, and it contains an API change that makes the system break when sending email from most extensions.
To address this issue, we have released CloudHotelier v3.1.10. We don't recommend upgrading to Joomla 3.5.x series yet, but if you did, you are covered with this patch. Thank you Giuliana for reporting the issue.
More info here: