When I enter the URL in my browser, it works perfectly: https://www.thecelticcello.com/events
When I put that URL in the Alternative Registration Page field and then click on the View Details button, I get a 403 Forbidden error on the other server.
Do you know why? Is there something that needs to be updated or changed on the celticcello website?
This looks like something that needs to be fixed within the core plugin. We can include the fix in an upcoming update of EE4.
What you could do in the meantime is change a setting in Event Espresso > Events > Templates. There’s a setting on that page, Display Ticket Selector, you can set that option to Yes and that will change the “View Details” button into a “Register Now” button and will also allow getting the information from your alternative registration page.