only to clarify the terms: Neither Your template nor the nx_youtubebox are plugins. The first is a template, the second a module. If You want to speak generally, please use the term extension. Joomla has multiple sections of extensions: Components (BwPostman is primarily a component), modules (BwPostman delivers some modules with the package), plugins (BwPostman also delivers some plugins with the package), templates, libraries, files… Each section of them have special use cases.
To be honest: There are myriads of templates and a similar number of video players for Joomla out there. Please forgive me, that I can't and won't install and test them all. Without a concrete and specific error description and a link to the not working site help is impossible.
I suppose, You have the module for registration to the newsletter active at the not working site. If this is the case, then deactivate the module and look, if the error is gone. Or do You have Your video player at a module position and the error appears at the registration page of BwPostman?