Hello,
This is because the JSON API responds with HTML rather than JSON. You are probably seeing some error.
What do you see in your browser when you navigate to https://www.example.com/?__wallets_apiversion=3&__wallets_action=get_coins_info ? (replace “www.example.com” with your domain name).
In production you should disable errors from being displayed in the frontend. Do this with define( 'WP_DEBUG_DISPLAY', false ); in your wp-config.php.
Additionally, it’s possible that some caching plugin is caching the JSON responses. See if disabling the caching plugin makes a difference. If it does, re-enable the plugin and white-list any urls with the __wallets_action GET parameter.
If disabling caching plugins does not make a difference, there should be some type of error in your WordPress debug log, since you mentioned that the cron job displays an incomplete page. Instructions for enabling debugging are here: https://wordpress.org/support/article/debugging-in-wordpress/
Hope this helps. Let me know what errors you see.
with regards