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
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I'm running Joomla v.3.4.6 php 5.5.30 CGI/FastCGI.What's weird is that's not really even our code. We just use JForms, and designate certain fields as being required. The J! code then automagically validates those fields.
Are you running J! 3.4.x? Not the 3.5 beta?
-- hugh
I've seen this sometimes, too. I thinks it's showing if trying to save the form before JS has finished loading."At least one group must be checked"