Every day I receive an email with contents similar to

The listname@example.com mailing list has 2
request(s) waiting for your consideration at:

        http://www.example.com/mailman/admindb/listname
        
Please attend to this at your earliest convenience.  This notice of
pending requests, if any, will be sent out daily.

but when I go to the URL, the page says

Database Updated...
listname Administrative Database
There are no pending requests. Click here to reload this page.

What is the reason for this?

The reason for this is ALWAYS that the notices are sent from a different Mailman instance. There is some test, backup or incompletely uninstalled Mailman installation that is sending these messages.

It may be on the same server as the production Mailman or another backup, test or 'migration' server.

You need to find the Mailman instance that is actually sending the message and at least remove the crontab that is running its cron/checkdbs job.

The same underlying situation can cause password reminders to be sent to unsubscribed members or can cause current members to receive 'duplicate' reminders. Another possible cause of duplicate password reminders is Mailman's cron/mailpasswds job being run from two crontabs, e.g. /etc/cron.d/mailman and /var/spool/cron/mailman.

The first step is to look at the Received: headers of the notices which should identify the IP address of the originating server.