Symptoms
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Error is displayed in different Plesk extensions, e.g. on the Monitoring page or when publishing domains via SiteJet:
ERR_SSL_PROTOCOL_ERROR
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With authentication extension enabled (e.g. Multi-Factor Authentication (MFA) or Google Authenticator), accessing Plesk via login link redirects to port 8880:
https://example.com:8880/modules/mfa/index.php/index/auth?return
Cause
A known Plesk issue that is going to be fixed in the nearest update.
Resolution
As a workaround, until the bug is fixed, substitute port 8880 in URLs that return errors with 8443 like so:
https://example.com:8443/modules/mfa/index.php/index/auth?return