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.
It's working fine for me.
Did you set all options in
http://www.fabrikar.com/forums/index.php?account/preferences
http://www.fabrikar.com/forums/index.php?account/alert-preferences
I got an "Alert" but no email although my preferences are "and receive email notifications of replies"