This happens in magento2 when you migrate to the latest version i.e 2.4.5, it has some tightly coupled and strictly following structure in XML.
You need to configure and check that your XML template in HTML template is declared, which produces the error:
Exception #0 (Magento\Framework\Exception\ValidatorException): Invalid `template file: 'subscribe_popup.phtml' in module: 'Ebizmarts_MailChimp' block's name: 'porto_newsletter_popup'`
As of Magento 2.4.3 HTML tags must have nested and closed properly to meet standards in JQuery 3.5.x for non-void elements including custom elements.
So you have to make sure all *.html has the correct open/close tags.
You are facing an issue with the XML file layout structure.
To resolve this, please make sure as of Magento 2.4.5, the XML structure is quite strictly identified:
Steps to reproduce:
The component path to HTML is correct, i.e., The path to the
component's .html template. ui/form/field exists.
Every tag has an open and closed tag.
As of Magento 2.4.5, HTML tags must have nested and closed properly
to meet standards in JQuery 3.5.x for non-void elements, including
custom elements.
Please check your custom HTML element template
My_Module/element/input contains proper HTML with open and closed
tags to match the standards.
As in the latest release, the Magento KO engine is quite strict and complex with these things.
Also your theme porto and module Newsletter is not locating the template file that you define in the Block path.
'Ebizmarts_MailChimp' block's name: 'porto_newsletter_popup'
``
Make sure you have the correct template HTML located with subscribe module.