When submitting an event registration form, acknowledgement emails are not sent. When you look at the acknowledgement under Email, Acknowledgements, all the rows are blank.
This issue was caused by an invalid URL being saved in the site URL field. This site URL had a space after the domain which caused the authentication with the email server to fail, and an error was produced in the dbo.Error table. Correcting the site URL resolved the issue.
The Site URL http://www.domain.org / is not valid for sending emails. Please specify a fully qualified domain name for the Site URL within Sites & Settings.