In case you’re seeing 503 Service Unavailable Errors, this implies PHP processes are crashing while executing a script. Therefore, Apache is not getting a valid response from PHP and considers the PHP-FPM service to be unavailable. It means that the site has reached the limit on how many simultaneous “requests” it can handle from you.
The most common cause of this problem is buggy PHP code, such as buggy WordPress plugins or slow scripts. The best solution is to disable all WordPress plugins and check your app’s PHP error log and PHP slow log. This problem can also be caused by buggy third-party PHP extensions or your server running out of memory.
How to Fix the 503 Service Unavailable Error
Restart your router and modem
While the 503 error is still the error of the website you’re visiting, it’s possible that there’s an issue with the DNS server configuration on your router or computer, which a basic restart of both may correct it.
Retry the URL from the address bar again by clicking the reload or refresh button
Even “503 Service Unavailable Error” implies that there’s an error on another computer, the issue is probably temporary. Simply attempting the page to reload or refresh will work again.
Contact the WordPress Support directly for help.
WordPress Support already know about the “503 Service Unavailable Error” and checking the status of your wordpress is a best idea.