It could be internal server error and I can only verify it through FTP. Would you mind providing your FTP login credentials as well?
Do you have any custom code that filters the_content? It could be the reason since activating another theme fixes it. Custom code is usually under the child theme.
There was a file in there called single.php. I renamed it and the page started working. Thanks for the tip. I guess it will do no harm to remove that from the child theme right?