Hey Tomasz, good to read you. I hope you are fine and with good health.
I have validated the files.publii.json file using the link you provided an it resulted in a “Valid JSON” message as you can see in this screenshot.
For what is about the deployment method: yes. I am using the FTP protocol with SSL/TLS in the Server Settings as shown in the Server Settings page, you can see my configuration on the following screenshots. I have changed some sensible data just for the screenshot to avoid problems.
I am not using the “Use relative URLs” because of the warning shown on the foot of this option, which reads: Please remember: while using relative URLs, some features like Open Graph tags, sharing buttons etc. cannot work properly.
If you think this option may solve something just let me know. Please.