WordPress Issues: Internal Server Error
Internal Server Error’s
If a WordPress site is loading an Internal Server Error, it is most commonly caused by something in the .htaccess file.
A default WordPress .htaccess file will look like this:
If the .htaccess file has other information, I would start by renaming it to .htaccess-old.
Then create a new .htaccess file, and add the default WordPress info to the file.
This should correct the issue. At this point, I would advise the client that something in the .htaccess file was causing the issue, and that you have replaced it with the WordPress default info. Also let them know that you left the renamed copy for them incase there is any information in it that they need access to.