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The default 'admin' user is created during the initial installation with its default password 'admin_password'. This password, like for any UDMG user is encrypted in the database with the AES passphrase file that is referenced in the configuration filehashed and cannot be recovered.
If you have lost of forgotten the password for this administrator user (or any other user that was created for admin purpose), you can follow the following steps to reset it.
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