Could it be more explicit? I’ve been there for two days and nothing … what do we have to do?
I also use a Mutu Ovh. the web update crashed from 17 --> 18 and give a white page.
The updater give the error :
To login you need to provide the unhashed value of “updater.secret” in your config file
A friend gave me this tip to run command line update without SSH through a web page (no SSH with mutualisé OVH). This fixed and ended the installation.
https://MySite/nextcloud/core/doc/admin/configuration_server/occ_command.html#command-line-upgrade
<?php
echo '<pre>';
$last_line = system('/usr/local/php7.2/bin/php occ upgrade -v', $retval);
</pre>'
<hr />La dernière ligne en sortie de la commande : ' . $last_line . '
<hr />Valeur retournée : ' . $retval;
?>
Found this thread while troubleshooting my update as well. As it was the first thing to pop up for me, I might as well share my solution.
So to round things up: The file /config/config.php holds the attribute updater.secret which should look something like this:
'updater.secret' => '$2y$10$.vGA1O9wmRjrwAVXD98HNOgsNpDczlqm3Jq7KnEd1rVAGv3Fykk1a',
);
The value represents the updater password in hashed form. As the whole point of a one-way hash is that you can’t reverse it (easily), you’re best of by creating a new password and putting the hash in the config. That’s exactly what the provided command in the error message does.
However, if you don’t have command line access but file access (say via FTP), you can choose a new password, hash it online, e.g. via https://phppasswordhash.com/ and then put it into the config (either by downloading, modifying and uploading or by directly changing it on the server).
Hope that helps future people when browsing for a solution. 
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