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When the button Submit all data is clicked in a Mailman mailing list, the site reloads. Actions specified for Plesk emails in the mailing list are not applied. 
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When submitting data after entering anything in the Search field, the following error is shown: The information you’re about to submit is not secure 
 Because this form is being submitted using a connection that’s not secure, your information will be visible to others.
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In the Developer's console of a browser (F12) the following error can be found: Mixed Content: The page at 'https://lists.example.com/mailman/admin/list_name ' was loaded over a secure connection, but contains a form that targets an insecure endpoint 'http://lists.example.com/cgi-bin/mailman/admin/list_name /general'. This endpoint should be made available over a secure connection. 
Cause
Mailman misconfiguration.
Resolution
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Connect to the server via SSH. 
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Open the /usr/lib/mailman/Mailman/mm_cfg.pyfile in any text editor. For example, in the vi text editor:# vi /usr/lib/mailman/Mailman/mm_cfg.py 
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Find the line beginning with DEFAULT_URL_PATTERNand change it as follows:- 
For Debian-based operating systems: DEFAULT_URL_PATTERN = 'https://%s/cgi-bin/mailman/' 
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For RHEL-based operating systems: DEFAULT_URL_PATTERN = 'https://%s/mailman/' 
 Note: If the line doesn't exist, add it manually 
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Save changes and close the file. 
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Execute the command below to apply changes: # /usr/lib/mailman/bin/withlist -l -r fix_url list_name -u lists.example.com