I am facing this same problem the moment I purchase DTRegister till now v2.3.4. Have also opened a ticket and close off previously as there seems to be no solution from the developer side. A little disappointing.
My DTRegister is installed in the following setup:
i. Jommla! v1.5.3 Legacy Mode
ii. SiteTemplate from olwebdesign.com
iii. JEvents v1.4.3RC2
Have did some testing on my part, seems like it maybe a conflict with the site template and have tested a couple of templates from olwebdesign all facing the same problem.
However DTregister seems functioning using the default joomla template, rhuk_milkyway.
Would appreciate the developer team to solve this problem. Till date I still did not deploy my purchased dtregister license due to this major bug!
Many thanks.