I know that there was an issue with this failure in earlier revs.
This just started happening on my sight today. Nothing has changed on the site since the day before.
I am running v2.7.13a on Joomla 1.5.25.
Whenever one completes a payment transaction either through our auth.net gateway or the pay offline gateway, DT register displays a blank screen and hangs there. It is hanging when attempting to send conf emails. If I refresh the page link, I see the "Could not instantiate mail" error listed twice - I assume one each for the user and admin email transactions.
Problem seems to exist for all events now. As I said, the system was working fine yesterday... nothing has changed at my end. Not sure about server updates.
There are no bugs with this. It would only be settings related. Check your configuration and view the Emails tab. Make sure that "From Name" and "From Email" are setup. Maybe this was erased somehow??
Also make sure your Thanks Email / subject as well as the Admin Notification Email / subject are configured.
Also, do you have any DT related plugins installed maybe that should not be? JomSocial plugins without having the JomSocial component maybe?
When you have a blank screen, it means there is probably a fatal error. Set your error reporting in Joomla config to Maximum and see what the error says. That would help if you believe your config is all good.
If you can't find the source, submit a ticket. Thanks.