Checkout error "-1"

When I try to check out with Paypal Express, I get a blank page with “1” in the upper left corner. This error does not occur when I am logged in as an administrator.

My WordPress, theme, and all my plugins are up-to-date. I have tried disabling all plugins except Woocommerce and this plugin, but I get the same error (“1” on blank page). I enabled error logging and no errors are being logged in the error log file. I am using the Woocommerce Storefront theme. My web host is WP Engine.

Any ideas how to resolve this problem?

https://ww.wp.xz.cn/plugins/paypal-for-woocommerce/