Page caching is an action that browsers perform to prevent from having to fetch every resource every time. This greatly reduces the amount of bandwidth that your computer uses, but it also has it's side effects.
A page may not be updated immediately after changing the content via FTP, cPanel File Manager, or other means. The time for the updated page to be displayed depends on the method of transfer, as well as the user's browser settings.
The cPanel File Manager usually has a slight delay of 5 minutes - during those 5 minutes it usually shows 500 Internal Server Errors to anyone accessing the page. FTP or Web Disk is the best way to upload and update pages quickly.
Also, the user's browser may cache some pages for quick loading. You can prevent this by adding this to your HTML code, between <head> and </head>:
<meta http-equiv="Cache-Control" content="no-store" />
Also, another way that has helped rather than clearing the cache for the entire web browser, you can just push Ctrl + F5 and it forces a refresh of that page which has helped with the problems.