My web hosting is erufa.com, and I host the wordpress site myself by its built-it services.
Moreover, I should have said that "only admin pages have the 500 server errors". And content pages became like this:
Not Found
The requested URL was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Could I know what should I do or where to seek help? Really grateful.