Can't log when using updater on web Authentification failed

Nextcloud version _: 15.0.7.0
Operating system and version : Linux Debian 10
Apache : Apache/2.4.38 (Debian)
PHP version : PHP 7.3.4-2
The issue :
When I try to enter this URL
http://localhost/nextcloud/updater/index.php
Authentification is not possible, not enough characters for entry form is too short I can’t enter the entire value of update.secret
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
I can refresh the login page.
Subsidiary question :
How to access admin screen ? I didn’t find any information about it in administration manual.
The output of your Nextcloud log in Admin > Logging:
No entry in nexcloud log for this trick.

The output of your Apache/nginx/system log in /var/log/____:

::1 - - [01/Jun/2019:09:09:12 +0200] "POST /nextcloud/updater/index.php HTTP/1.1" 200 3797 "-" "Mozilla/5.0 (Windows NT 6.1; rv:60.0) Gecko/20100101 Firefox/60.0"