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
See Announcement
We have resolved the issue with the J! updater and this will be fixed in the next release.
Dunno, I just tried editing and submitting a form on that menu item you pointed to.
If you got a "Syntax error", that probably means you were submitting it through AJAX, and the error I saw was being returned in the JSON response.
And just to be clear, you cannot edit or create forms on a list built on a view. They are "read only".