TOPIC: out of date bookmark

after creating an event and testing it, we can go through the whole booking, choose our method of payment, when we click the 'next step' button to finalise it and are redirected to a page that says 'You may not be able to visit this page because of'

1.an out-of-date bookmark/favourite
2.a search engine that has an out-of-date listing for this site
3.a mistyped address
4.you have no access to this page
5.The requested resource was not found.
6.An error has occurred while processing your request.

We are running Joomla 3.5.1 with DTRegister 3.1.8 and DTInvoice 1.0.1.
PHP Version 5.6.18 SEF URL rewriting is turned off.

We have used DTRegister for over 3 years now without any problems, I've had to revert back to our old website because I've had so many buggy problems with it on the new version, some have been corrected through settings and human error on our end. DTInvoice has been configured but doesn't work at all. It could very well be settings our end, but could you please advise as to where you think the problem may lie?

Thanks