Symptoms
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Plesk server was secured with a Let's Encrypt certificate via Tools & Settings > SSL/TLS Certificates > + Let's Encrypt button
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The FTP site in IIS is still using the default self-signed certificate, as can be seen in Internet Information Services (IIS) Manager > Sites > server's IP address > FTP SSL Settings:
Cause
This is expected behavior. Plesk does not secure FTP sites on Windows.
Resolution
Installing Let's Encrypt certificate for FTP sites is yet to be implemented in Plesk.
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As a workaround, it is possible to manually assign a certificate to the FTP site:
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Navigate to Tools & Settings > IP Addresses and select the Plesk server's IP
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Select a certificate from the dropdown list in SSL/TLS certificate section:
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Click OK
Warning: the certificate for FTP site will not be auto-renewed, it should be renewed manually