[Theme Horse Support Team]

Hi Hellen,

Can you please deactivate all the plugins and check once.
Plugins may cause such issue as well.

Just go back to previous version of the PHP and deactivate all the plugins and again upgrade to the PHP 7.1 OR Here is how to deactivate all plugins when not able to access the backend

• Via FTP or your host’s file manager, navigate to the wp-contents folder (directory)
• rename the folder “plugins” to “plugins.hold”
• Login to your WordPress administration plugins page (/wp-admin/plugins.php) – this will disable any plugin that is “missing”.
• rename “plugins.hold” back to “plugins”

Thank you!