As other people have found, switching from PHP 5.x to 7.x fixed the issue for me. There was one caveat though…
In cPanel, you can use the MultiPHP Manager to change the version of PHP that the site runs on. But, if you use WP-CLI, then it will still run on the default system version. So if that default version is still PHP 5.x, you’ll continue to see the error.
In that case, run this command and it will tell you what version of PHP it’s using.
wp-cli --info
Follow either of these links in order to make wp-cli use a different version of PHP:
https://make.ww.wp.xz.cn/cli/handbook/installing/#using-a-custom-php-binary
https://wordpress.stackexchange.com/questions/244164/wp-cli-selecting-php-version
https://stackoverflow.com/questions/31206864/use-different-php-version-cli-executable-for-one-command
I had the same issue with both testimonials and WPML plugin installed.
I found the answer in the WPML support forum:
https://wpml.org/forums/topic/posts-disappear-when-the-plugin-is-activated/
The fix for me was to go to the WPML Support page, click the troubleshooting link, and then click the Set Language Information button.
