After installing the suggested plugins in the setup wizard, the user is asked specify admin user credentials and name.

None of these fields is marked as required, but leaving the E-Mail address field empty freezes the setup wizard after hitting Save and Finish.

The request triggers the following exception output:

Jul 09, 2017 1:56:38 PM hudson.init.impl.InstallUncaughtExceptionHandler$1 reportException
WARNING: null
java.io.IOException: finished
 at com.jcraft.jzlib.DeflaterOutputStream.write(DeflaterOutputStream.java:90)

As the setup wizard disables all buttons, the only chance for the user is to reload the page (which allows the user to think again and enter more values and finally successfully finish the wizard).

While the POST request to 

/setupWizard/createAdminUser

is actually responded with 200 OK, its HTML body containing the message Invalid e-mail address is not displayed to the user (only visible using the developer tools).