Thank you Troester and Trembowiecki ! I confirm the same issue with the French language in the back-end, which is now solved with an english language back-end for my case...
Thank you the team, Henk, Alex, Beate, excellent job Fabrik is a great really great extention. I use it all days, on J4 anf J5 now, and the mix of Fabrik and MySql views/triggers is for me really perfect to build powerfull websites.
My environment
Joomla 3.11
Fabrik latests version from Github
PHP 8.0 or 7.4
I use a form for User registration with the plugin JUser.
With PHP 7.4, no problems at all.
With PHP 8.0, in the database / table users, the fields name and username are just empty, without any error notification ...