There's quite a few steps in order to "ajaxify" things:
That's pretty much it I think. See the message board core plugin for an example.
As I replied on your previous post on the Loginrequired plugin page it would be necessary to know what additional 3rd party plugins you are using the does this form validation on the register page. The reason for the error is very likely due to this plugin (or maybe even plugin if there's more than one involved) not working correctly within a walled-garden Elgg site (neither Elgg core walled-garden or with the Loginrequired plugin). For the plugin to work with walled-garden enabled it would be necessary to allow access to certain pages or actions this plugin provides when not logged in. But it's just impossible to say which one these are without even knowing what plugin(s) are involved.
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