This can happen due to a permissions issue. Make sure all the PHP files have 644 permission and all the folders have 755 permission.
Also, If you are getting 500 error while exporting or importing Excel file then make sure your server meets following requirements.
- PHP extension php_zip enabled
- PHP extension php_xml enabled
- PHP extension php_gd2 enabled (if not compiled in)
- PHP extension php_mbstring enabled