Is the Presentation URL for the site accessible? There should also be additional information about the error somewhere, can you please check the browser's Console and Network tab? If there are failing requests, are there any details in the request that help? Is there anything in the Event log application which would be related to the error?
Also, if your administration and Core site run on different domains, make sure they both run on HTTPS and you have the SameSite cookie mode set to None according to documentation - Configuring cookie SameSite mode.
Then, I would also run the code in debug mode and make sure you have registered the page templates correctly. What is the code of your page templates?