Warning: Undefined variable $zone_text in /var/www/html/docroot/modules/contrib/cloudflare/src/Form/SettingsForm.php on line 258

We hit this as an edge case related to loading in config values via environment variables.

The attached patch just ensures variables are created before they're used, and also skips processing if $zones cannot be populated from the Cloudflare API.

There are still some deeper issues involving a mismatch between invalid credentials but $config['cloudflare.settings']['valid_credentials'] = TRUE that can trigger WSODs, but that seems to be contained within the cloudflare/sdk rather than this module directly ― and to be fair it's probably just a result of bad config.

Error is:
Cloudflare\API\Adapter\ResponseException: Invalid request headers in Cloudflare\API\Adapter\ResponseException::fromRequestException() (line 38 of /app/vendor/cloudflare/sdk/src/Adapter/ResponseException.php).

This also affects us in 2.0.0-beta1.
And I agree with @jrsouth that the underlying problem ist some authentication / authorization issue with the Cloudflare API which brings up this error. On one hand this actual error shall be fixed but on the other hand some information about invalid Cloudflare API communication may be helpful to a user.