When editing a Registration and choosing a Joomla user to attach to it, the window to find a User comes up, I search, and then click on the User - nothing happens. I then bring up the search again and on the 2nd time clicking the user, the field does fill with the name. Not sure why you have to do this 2x, but seems like a glitch.
We actually use Joomla modal input for that. I will have to check it again. If the issue happens and easy to re-procedure, I am going to fix it in the next release of the extension