(Temporary Fix) “Error for site owner: Required field must not be blank”

This is a problem mainly with Recaptcha on Chrome & Safari.

You can download a temporary fix for here [Tested]:
http://averyniceweb.com/support/download/um-recaptcha.zip

Once in your computer, go to the Admin Panel > Plugins > add new.
Install & activate.

Refresh your registration page (press Shift + F5).

If you are unsure that you want to do it without precautions, you can follow the instructions for a no risk installation:
http://averyniceweb.com/support/viewtopic.php?f=14&t=89

Otherwise, this fix should work until UM releases a new version of the plugin with a permanent fix.