We addressed this through a ticket, but as the issue is double-posted here, we'll answer here too. This wasn't actually a DT Register problem. It was due to template overrides that had been setup.
In 2.7.14, all session handling was revamped so that required updates to many files. This site was pulling in old DT Reg files that were missing the necessary session-related changes. We applied the needed changes to those files and all is well now.
For anyone that does these type of overrides to make your own customizations to the look of anything in DT Reg... If you ever update and see odd issues like this user did, the first thing you should check is the new package to see if the timestamp on any of the template files that you have modified versions of have been updated. If so, you'll need to get the updated version in place. You can't assume those files will never be modified