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.
Is this a Bootstrap2 template?I use a template created with "Template Creator CK V3"
What were you using before (Fabrik3.3.2/Joomla3.3+ or e.g. Fabrik3.0.9/Joomla2.5.x)?Since the update Fabrikar 3.3.3