The exact cause is quite clear, but what is leading to that cause is unusual.If it helps: I deactivated the add-on, then there are no server errors logged when someone likes the picture in the gallery.
As soon as I activate the add-on, the server errors occur again the same way. It seems like the initiation of the push itself does generate the server error.
The exact cause is I try to render the content of the alert template, which should return some HTML. For some reason, rendering the HTML is blank, therefore we get this no document registered error.
I just can't work out why the rendering of the alert template would fail.
Is this something you would be happy for me to debug on your server? I will need FTP access and Admin CP access to do this.