WP Smush Pro - 'Request failed Site URL not accessible'

Dear support,
I'm having this error when trying to smush single images.
I don' t have any security plugin installed and I already disabled WP Super Cache.
If I check the System Info I see:
---
Remote HTTP Requests
WPMU DEV: GET OK
WPMU DEV: POST OK
PayPal API: POST OK
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Support access is now granted.

Thanks for your help