When your website for some reason returns codes 5** or explicitly displays an error message on the page, the error log, aka error_log file, can help you diagnose the problem. It stores the history of PHP errors.
There is no established format, because errors are generated by a particular script: therefore, the errors may be super-detailed or just bland text without much detail.
But in any case, the error message will be able to shed some truth and help you find the reasons for this behaviour: either through documentation or through Google search ? .
You can find the access log in the hosting control panel, section "System info and files" ➡️ "Site summary / Statistics / Logs". In the section will be a list of all domains and subdomains in the hosting account, and next to each one of them will be a column "logs". Click on "error log".