500 errors are caused by incorrect website configuration. There are several reasons for this.

What causes 500 Internal Server Errors?

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.htaccess

If the .htaccess syntax is incorrect, the web server will return a 500 Internal Server Error. To confirm whether .htaccess is the cause, rename the .htaccess file .htaccess_bak for example.

Find out more:All about the .htaccess file
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Permissions:

You must follow a few security rules concerning the permissions you give to your scripts.
  • your website's root folder must be set to 705 (the default permissions set up by OVH). This is your FTP server / (slash) or . (point) directory, do not change it.
  • other directories must be set to no higher than 755,
  • php/cgi scripts must be set to no higher than 755.
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Script error

If you program in perl for example, an error in your script is summed up by a 500 error. Due to security you will not be given more details. To debug your scripts, you can use the telnet/ssh connection (available with Professional hosting and upwards).

Click here for more information: Web hosting: SSH on web hosting packages