For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
I certainly don't want users to encourage to delete their registration but I think it is nessecary by law, isn't it?but I am not sure why you want to encourage your users to delete their registration?
Have you watched the video tutorial on the Fabrik site?