Hello.
Do you mean when you click "Add document type" button on Child types tab? It is weird, it can be some environment-related issue, since I am not able to reproduce it on my machine (and I can confirm it is not a bug).
Would it be possible to get access to your
CMS, so I can investigate it there? If so, please send us an e-mail to support@kentico.com and refer to this forum thread. Thank you.
Best Regards,
Radek Macalik