This may be caused by a problem with the SMTP server. Actinic sends the emails at the same time as returning the Receipt Page. If sending the email causes a serious error the Receipt Page is not delivered.
This can be checked by temporarily removing the ticks from the tick boxes 'Send me an Email When I have an Order to Download' and 'Send Receipt Email to Customer' in 'Settings | Business Settings | Ordering' * , then updating the site and placing a test order. If the Receipt Page is now displayed then the SMTP server was probably the cause of the problem, examining the file error.err in the acatalog folder on the server might indicate the nature of the SMTP problem.
Please see the article here on SMTP problems.
This can be checked by temporarily removing the ticks from the tick boxes 'Send me an Email When I have an Order to Download' and 'Send Receipt Email to Customer' in 'Settings | Business Settings | Ordering' * , then updating the site and placing a test order. If the Receipt Page is now displayed then the SMTP server was probably the cause of the problem, examining the file error.err in the acatalog folder on the server might indicate the nature of the SMTP problem.
Please see the article here on SMTP problems.