503 Service Unavailable message on a single, upper level page
A temporary solution you can try. Create a new page with the content of the page that is causing your problem.
I encountered this problem last year on a test site. Not on one or two pages, but on the entire tiki site. After several days of searching here and there, even reverting to the previous version, the 503 error remained. I solved my problem by changing hosting providers. I always make a backup of the database and all the website files.
What I noticed was that the MySQL version was obsolete on the old hosting provider. Once the domain name, database, and all the files were migrated, the problem was solved. But I don't think that's a solution in your case.
Your website is hosted by hostgator and you can find help here https://www.hostgator.com/help/article/503-service-unavailable-error-message
I think a Hostgator have a good support. Maybe a solution!!