We found this users problem through their ticket. Not a DT Register problem, but an issue caused by another extension. Here's the details:
The problem is due to the Server Statistics Admin module (mod_minisystem). Never seen or heard of this module before. When I turned error reporting on maximum, your Joomla control panel showed lots of PHP warnings from that module. Not sure what it does, but it obviously affects the whole backend... not in a good way. I disabled that module and DT Register worked just fine. I saved the current event and also created a new one. That module seems to also have been contributing to the slow admin loading. Seemed to load faster after I disabled it.
So... if anyone else has this module (which we've not seen on anyone's site before), you may want to re-consider using it. Or maybe there is a fix for that module? An update?